Tune TiFlash Performance
This document introduces how to tune the performance of TiFlash by properly planning machine resources and tuning TiDB parameters. By following these methods, your TiFlash cluster can achieve optimal performance.
Plan resources
If you want to save machine resources and have no requirement on isolation, you can use the method that combines the deployment of both TiKV and TiFlash. It is recommended that you save enough resources for TiKV and TiFlash respectively, and do not share disks.
Tune TiDB parameters
This section describes how to improve TiFlash performance by tuning TiDB parameters, including:
- Forcibly enable the MPP mode
- Push down aggregate functions to a position before
JoinorUnion - Enable
Distinctoptimization - Compact data using the
ALTER TABLE ... COMPACTstatement - Replace Shuffled Hash Join with Broadcast Hash Join
- Set a greater execution concurrency
- Configure
tiflash_fine_grained_shuffle_stream_count
Forcibly enable the MPP mode
MPP execution plans can fully utilize distributed computing resources, thereby significantly improving the efficiency of batch data queries. When the optimizer does not generate an MPP execution plan for a query, you can forcibly enable the MPP mode:
The variable tidb_enforce_mpp controls whether to ignore the optimizer's cost estimation and to forcibly use TiFlash's MPP mode for query execution. To enable MPP mode forcibly, run the following command:
set @@tidb_enforce_mpp = ON;
The following example shows the query result before and after tidb_enforce_mpp is enabled. Before this variable is enabled, TiDB needs to read data from TiKV and execute Join and Aggregation in TiDB. After tidb_enforce_mpp is enabled, Join and Aggregation are pushed down to TiFlash. In addition, because the optimizer does not necessarily generate MPP execution plans, by enabling tidb_enforce_mpp, you can force the optimizer to generate MPP execution plans.
Before MPP mode is enabled:
mysql> explain analyze select o_orderpriority, count(*) as order_count from orders where o_orderdate >= '1995-01-01' and o_orderdate < date_add('1995-01-01', interval '3' month) and exists (select * from lineitem where l_orderkey = o_orderkey and l_commitdate < l_receiptdate) group by o_orderpriority;
+--------------------------------------+--------------+-----------+-----------+---------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+--------------------------------------+--------------+-----------+-----------+---------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------+------+
| Projection_9 | 1.00 | 5 | root | | time:22.8s, loops:5, Concurrency:OFF | tpch_100.orders.o_orderpriority, Column#27 | 1016 Bytes | N/A |
| └─HashAgg_10 | 1.00 | 5 | root | | time:22.8s, loops:5, partial_worker:{wall_time:22.817403422s, concurrency:5, task_num:5027, tot_wait:1m52.560509138s, tot_exec:1.523754659s, tot_time:1m54.086196522s, max:22.817363394s, p95:22.817363394s}, final_worker:{wall_time:22.817416046s, concurrency:5, task_num:20, tot_wait:1m54.086784741s, tot_exec:155.94µs, tot_time:1m54.086948982s, max:22.817396284s, p95:22.817396284s} | group by:tpch_100.orders.o_orderpriority, funcs:count(1)->Column#27, funcs:firstrow(tpch_100.orders.o_orderpriority)->tpch_100.orders.o_orderpriority | 511.7 KB | N/A |
| └─IndexHashJoin_18 | 4429262.13 | 5145138 | root | | time:22.8s, loops:5028, inner:{total:1m52.6s, concurrency:5, task:227, construct:5.17s, fetch:1m40s, build:1.22s, join:7.49s} | semi join, inner:IndexLookUp_15, outer key:tpch_100.orders.o_orderkey, inner key:tpch_100.lineitem.l_orderkey, equal cond:eq(tpch_100.orders.o_orderkey, tpch_100.lineitem.l_orderkey) | 46.8 MB | N/A |
| ├─TableReader_37(Build) | 5536577.67 | 5610951 | root | | time:4.89s, loops:5467, cop_task: {num: 2127, max: 408.7ms, min: 277.5µs, avg: 81.6ms, p95: 249.7ms, max_proc_keys: 226272, p95_proc_keys: 220128, tot_proc: 2m50.2s, tot_wait: 40ms, rpc_num: 2127, rpc_time: 2m53.6s, copr_cache_hit_ratio: 0.02, distsql_concurrency: 15} | data:Selection_36 | 3.42 MB | N/A |
| │ └─Selection_36 | 5536577.67 | 5610951 | cop[tikv] | | tikv_task:{proc max:402ms, min:0s, avg: 79.7ms, p80:151ms, p95:245ms, iters:155122, tasks:2127}, scan_detail: {total_process_keys: 149478336, total_process_keys_size: 22705157593, total_keys: 149480413, get_snapshot_time: 187.9ms, rocksdb: {key_skipped_count: 149478336, block: {cache_hit_count: 393690}}} | ge(tpch_100.orders.o_orderdate, 1995-01-01 00:00:00.000000), lt(tpch_100.orders.o_orderdate, 1995-04-01 00:00:00.000000) | N/A | N/A |
| │ └─TableFullScan_35 | 150000000.00 | 150000000 | cop[tikv] | table:orders | tikv_task:{proc max:382ms, min:0s, avg: 74.6ms, p80:141ms, p95:229ms, iters:155122, tasks:2127} | keep order:false | N/A | N/A |
| └─IndexLookUp_15(Probe) | 22018496.89 | 14185840 | root | | time:1m33.4s, loops:14204, index_task: {total_time: 1m10.8s, fetch_handle: 1m10.7s, build: 3.9ms, wait: 13.3ms}, table_task: {total_time: 1m44.4s, num: 1989, concurrency: 5} | | 17.9 KB | N/A |
| ├─IndexRangeScan_12(Build) | 27523121.11 | 22441356 | cop[tikv] | table:lineitem, index:PRIMARY(L_ORDERKEY, L_LINENUMBER) | time:1m8.8s, loops:23635, cop_task: {num: 12537, max: 179.3ms, min: 331.4µs, avg: 22.7ms, p95: 66.9ms, max_proc_keys: 13755, p95_proc_keys: 5088, tot_proc: 3m44s, tot_wait: 129ms, rpc_num: 12537, rpc_time: 4m44s, copr_cache_hit_ratio: 0.06, distsql_concurrency: 15}, tikv_task:{proc max:170ms, min:0s, avg: 18.8ms, p80:29ms, p95:62ms, iters:68823, tasks:12537}, scan_detail: {total_process_keys: 21985403, total_process_keys_size: 1165226359, total_keys: 27492546, get_snapshot_time: 563.1ms, rocksdb: {key_skipped_count: 21985403, block: {cache_hit_count: 44004354}}} | range: decided by [eq(tpch_100.lineitem.l_orderkey, tpch_100.orders.o_orderkey)], keep order:false | N/A | N/A |
| └─Selection_14(Probe) | 22018496.89 | 14185840 | cop[tikv] | | time:1m34.8s, loops:16882, cop_task: {num: 12057, max: 240.4ms, min: 328.5µs, avg: 20.4ms, p95: 73.1ms, max_proc_keys: 15005, p95_proc_keys: 6118, tot_proc: 3m39.8s, tot_wait: 300ms, rpc_num: 12057, rpc_time: 4m5.7s, copr_cache_hit_ratio: 0.28, distsql_concurrency: 15}, tikv_task:{proc max:230ms, min:0s, avg: 27.5ms, p80:45ms, p95:82ms, iters:74165, tasks:12057}, scan_detail: {total_process_keys: 19916937, total_process_keys_size: 3957799619, total_keys: 24053143, get_snapshot_time: 1.07s, rocksdb: {key_skipped_count: 19254567, block: {cache_hit_count: 37816908}}} | lt(tpch_100.lineitem.l_commitdate, tpch_100.lineitem.l_receiptdate) | N/A | N/A |
| └─TableRowIDScan_13 | 27523121.11 | 22441356 | cop[tikv] | table:lineitem | tikv_task:{proc max:230ms, min:0s, avg: 27.2ms, p80:45ms, p95:81ms, iters:74165, tasks:12057} | keep order:false | N/A | N/A |
+--------------------------------------+--------------+-----------+-----------+---------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------+------+
10 rows in set (22.82 sec)
Enable MPP mode:
mysql> set @@tidb_enforce_mpp = ON;
Query OK, 0 rows affected (0.00 sec)
After MPP mode is enabled:
mysql> explain analyze select o_orderpriority, count(*) as order_count from orders where o_orderdate >= '1995-01-01' and o_orderdate < date_add('1995-01-01', interval '3' month) and exists (select * from lineitem where l_orderkey = o_orderkey and l_commitdate < l_receiptdate) group by o_orderpriority;
+--------------------------------------------------+--------------+-----------+--------------+----------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+--------------------------------------------------+--------------+-----------+--------------+----------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------+
| TableReader_59 | 1.00 | 5 | root | | time:5.97s, loops:2, cop_task: {num: 8, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_58 | N/A | N/A |
| └─ExchangeSender_58 | 1.00 | 5 | mpp[tiflash] | | tiflash_task:{proc max:5.97s, min:5.97s, avg: 5.97s, p80:5.97s, p95:5.97s, iters:5, tasks:3, threads:60} | ExchangeType: PassThrough | N/A | N/A |
| └─Projection_10 | 1.00 | 5 | mpp[tiflash] | | tiflash_task:{proc max:5.97s, min:5.96s, avg: 5.97s, p80:5.97s, p95:5.97s, iters:5, tasks:3, threads:60} | tpch_100.orders.o_orderpriority, Column#27 | N/A | N/A |
| └─Projection_54 | 1.00 | 5 | mpp[tiflash] | | tiflash_task:{proc max:5.97s, min:5.96s, avg: 5.97s, p80:5.97s, p95:5.97s, iters:5, tasks:3, threads:60} | Column#27, tpch_100.orders.o_orderpriority | N/A | N/A |
| └─HashAgg_55 | 1.00 | 5 | mpp[tiflash] | | tiflash_task:{proc max:5.97s, min:5.96s, avg: 5.97s, p80:5.97s, p95:5.97s, iters:5, tasks:3, threads:60} | group by:tpch_100.orders.o_orderpriority, funcs:sum(Column#32)->Column#27, funcs:firstrow(tpch_100.orders.o_orderpriority)->tpch_100.orders.o_orderpriority, stream_count: 20 | N/A | N/A |
| └─ExchangeReceiver_57 | 1.00 | 15 | mpp[tiflash] | | tiflash_task:{proc max:5.97s, min:5.96s, avg: 5.97s, p80:5.97s, p95:5.97s, iters:5, tasks:3, threads:60} | stream_count: 20 | N/A | N/A |
| └─ExchangeSender_56 | 1.00 | 15 | mpp[tiflash] | | tiflash_task:{proc max:5.96s, min:0s, avg: 1.99s, p80:5.96s, p95:5.96s, iters:768, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: tpch_100.orders.o_orderpriority, collate: utf8mb4_bin], stream_count: 20 | N/A | N/A |
| └─HashAgg_52 | 1.00 | 15 | mpp[tiflash] | | tiflash_task:{proc max:5.96s, min:0s, avg: 1.99s, p80:5.96s, p95:5.96s, iters:768, tasks:3, threads:60} | group by:tpch_100.orders.o_orderpriority, funcs:count(1)->Column#32 | N/A | N/A |
| └─HashJoin_41 | 4429262.13 | 5145138 | mpp[tiflash] | | tiflash_task:{proc max:5.95s, min:0s, avg: 1.98s, p80:5.95s, p95:5.95s, iters:118, tasks:3, threads:60} | semi join, equal:[eq(tpch_100.orders.o_orderkey, tpch_100.lineitem.l_orderkey)], stream_count: 20 | N/A | N/A |
| ├─ExchangeReceiver_26(Build) | 480030321.60 | 379356474 | mpp[tiflash] | | tiflash_task:{proc max:3.26s, min:0s, avg: 1.09s, p80:3.26s, p95:3.26s, iters:42469, tasks:3, threads:60} | stream_count: 20 | N/A | N/A |
| │ └─ExchangeSender_25 | 480030321.60 | 379356474 | mpp[tiflash] | | tiflash_task:{proc max:3.37s, min:0s, avg: 1.12s, p80:3.37s, p95:3.37s, iters:9292, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: tpch_100.lineitem.l_orderkey, collate: binary], stream_count: 20 | N/A | N/A |
| │ └─Projection_60 | 480030321.60 | 379356474 | mpp[tiflash] | | tiflash_task:{proc max:819.1ms, min:0s, avg: 273ms, p80:819.1ms, p95:819.1ms, iters:9292, tasks:3, threads:60} | tpch_100.lineitem.l_orderkey | N/A | N/A |
| │ └─Selection_24 | 480030321.60 | 379356474 | mpp[tiflash] | | tiflash_task:{proc max:817.1ms, min:0s, avg: 272.4ms, p80:817.1ms, p95:817.1ms, iters:9292, tasks:3, threads:60} | lt(tpch_100.lineitem.l_commitdate, tpch_100.lineitem.l_receiptdate) | N/A | N/A |
| │ └─TableFullScan_23 | 600037902.00 | 600037902 | mpp[tiflash] | table:lineitem | tiflash_task:{proc max:328.1ms, min:0s, avg: 109.4ms, p80:328.1ms, p95:328.1ms, iters:9292, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73475, total_skipped_packs:13452, total_scanned_rows:600230532, total_skipped_rows:109887612, total_rs_index_load_time: 35ms, total_read_time: 34065ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
| └─ExchangeReceiver_22(Probe) | 5536577.67 | 5610951 | mpp[tiflash] | | tiflash_task:{proc max:2.5s, min:0s, avg: 833.7ms, p80:2.5s, p95:2.5s, iters:669, tasks:3, threads:60} | | N/A | N/A |
| └─ExchangeSender_21 | 5536577.67 | 5610951 | mpp[tiflash] | | tiflash_task:{proc max:5.93s, min:0s, avg: 1.98s, p80:5.93s, p95:5.93s, iters:2336, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: tpch_100.orders.o_orderkey, collate: binary] | N/A | N/A |
| └─Selection_20 | 5536577.67 | 5610951 | mpp[tiflash] | | tiflash_task:{proc max:325.5ms, min:0s, avg: 108.5ms, p80:325.5ms, p95:325.5ms, iters:2336, tasks:3, threads:60} | ge(tpch_100.orders.o_orderdate, 1995-01-01 00:00:00.000000), lt(tpch_100.orders.o_orderdate, 1995-04-01 00:00:00.000000) | N/A | N/A |
| └─TableFullScan_19 | 150000000.00 | 150000000 | mpp[tiflash] | table:orders | tiflash_task:{proc max:238.5ms, min:0s, avg: 79.5ms, p80:238.5ms, p95:238.5ms, iters:2336, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:18541, total_skipped_packs:186, total_scanned_rows:150000611, total_skipped_rows:1513328, total_rs_index_load_time: 75ms, total_read_time: 11454ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+--------------------------------------------------+--------------+-----------+--------------+----------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------+
18 rows in set (6.00 sec)
Push down aggregate functions to a position before Join or Union
By pushing down aggregate operations to the position before Join or Union, you can reduce the data to be processed in the Join or Union operation, thereby improving performance.
The variable tidb_opt_agg_push_down controls whether the optimizer executes the optimization operation of pushing down the aggregate function to the position before Join or Union. When the aggregate operations are quite slow in the query, you can set this variable to ON.
set @@tidb_opt_agg_push_down = ON;
The following example shows the query result before and after the tidb_opt_agg_push_down variable is enabled. Before this variable is enabled, the HashAgg_58 operation is executed following the HashJoin_41 operation. After this variable is enabled, newly generated HashAgg_21 and HashAgg_32 operations are executed before the HashJoin_76 operation. This significantly reduces the data to be processed by the Join operation.
Before tidb_opt_agg_push_down is enabled:
mysql> explain analyze select count(*) from t1 join t2 where t1.a = t2.b group by t1.a;
+------------------------------------------+--------------+-----------+--------------+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------+--------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+------------------------------------------+--------------+-----------+--------------+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------+--------+------+
| TableReader_66 | 100450000.00 | 20 | root | | time:2.13s, loops:2, cop_task: {num: 21, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_65 | N/A | N/A |
| └─ExchangeSender_65 | 100450000.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:2.13s, min:2.08s, avg: 2.11s, p80:2.13s, p95:2.13s, iters:768, tasks:3, threads:60} | ExchangeType: PassThrough | N/A | N/A |
| └─Projection_60 | 100450000.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:2.13s, min:2.08s, avg: 2.11s, p80:2.13s, p95:2.13s, iters:768, tasks:3, threads:60} | Column#43 | N/A | N/A |
| └─HashAgg_58 | 100450000.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:2.13s, min:2.08s, avg: 2.11s, p80:2.13s, p95:2.13s, iters:768, tasks:3, threads:60} | group by:test.t1.a, funcs:count(1)->Column#43 | N/A | N/A |
| └─HashJoin_41 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:1.77s, min:1.67s, avg: 1.73s, p80:1.77s, p95:1.77s, iters:10955, tasks:3, threads:60} | inner join, equal:[eq(test.t1.a, test.t2.b)], stream_count: 20 | N/A | N/A |
| ├─ExchangeReceiver_22(Build) | 100450000.00 | 99980000 | mpp[tiflash] | | tiflash_task:{proc max:171ms, min:127ms, avg: 154.5ms, p80:171ms, p95:171ms, iters:10440, tasks:3, threads:60} | stream_count: 20 | N/A | N/A |
| │ └─ExchangeSender_21 | 100450000.00 | 99980000 | mpp[tiflash] | | tiflash_task:{proc max:498.7ms, min:0s, avg: 166.2ms, p80:498.7ms, p95:498.7ms, iters:1653, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: test.t1.a, collate: binary], stream_count: 20 | N/A | N/A |
| │ └─Selection_20 | 100450000.00 | 99980000 | mpp[tiflash] | | tiflash_task:{proc max:83.7ms, min:0s, avg: 27.9ms, p80:83.7ms, p95:83.7ms, iters:1653, tasks:3, threads:60} | not(isnull(test.t1.a)) | N/A | N/A |
| │ └─TableFullScan_19 | 100450000.00 | 99980000 | mpp[tiflash] | table:t1 | tiflash_task:{proc max:41.7ms, min:0s, avg: 13.9ms, p80:41.7ms, p95:41.7ms, iters:1653, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:12336, total_skipped_packs:5759, total_scanned_rows:100083566, total_skipped_rows:46786721, total_rs_index_load_time: 5ms, total_read_time: 1105ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
| └─ExchangeReceiver_26(Probe) | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:373ms, min:71ms, avg: 253.2ms, p80:373ms, p95:373ms, iters:28135, tasks:3, threads:60} | | N/A | N/A |
| └─ExchangeSender_25 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:2.07s, min:0s, avg: 689.6ms, p80:2.07s, p95:2.07s, iters:9388, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: test.t2.b, collate: binary] | N/A | N/A |
| └─Selection_24 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:146.9ms, min:0s, avg: 49ms, p80:146.9ms, p95:146.9ms, iters:9388, tasks:3, threads:60} | not(isnull(test.t2.b)) | N/A | N/A |
| └─TableFullScan_23 | 600000000.00 | 600000000 | mpp[tiflash] | table:t2 | tiflash_task:{proc max:63.8ms, min:0s, avg: 21.3ms, p80:63.8ms, p95:63.8ms, iters:9388, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73834, total_skipped_packs:439, total_scanned_rows:600007458, total_skipped_rows:3553131, total_rs_index_load_time: 12ms, total_read_time: 270ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+------------------------------------------+--------------+-----------+--------------+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------+--------+------+
13 rows in set (2.15 sec)
Enable tidb_opt_agg_push_down:
mysql> set @@tidb_opt_agg_push_down = ON;
Query OK, 0 rows affected (0.00 sec)
After tidb_opt_agg_push_down is enabled:
mysql> explain analyze select count(*) from t1 join t2 where t1.a = t2.b group by t1.a;
+------------------------------------------------------+--------------+-----------+--------------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------+--------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+------------------------------------------------------+--------------+-----------+--------------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------+--------+------+
| TableReader_85 | 20.00 | 20 | root | | time:432.4ms, loops:2, cop_task: {num: 19, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_84 | N/A | N/A |
| └─ExchangeSender_84 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:431.1ms, min:429.3ms, avg: 430.4ms, p80:431.1ms, p95:431.1ms, iters:16, tasks:3, threads:60} | ExchangeType: PassThrough | N/A | N/A |
| └─Projection_83 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:430.1ms, min:429.3ms, avg: 429.7ms, p80:430.1ms, p95:430.1ms, iters:16, tasks:3, threads:60} | Column#43 | N/A | N/A |
| └─HashAgg_82 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:430.1ms, min:429.3ms, avg: 429.7ms, p80:430.1ms, p95:430.1ms, iters:16, tasks:3, threads:60} | group by:test.t1.a, funcs:count(Column#44)->Column#43, stream_count: 20 | N/A | N/A |
| └─ExchangeReceiver_78 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:430.1ms, min:429.3ms, avg: 429.7ms, p80:430.1ms, p95:430.1ms, iters:16, tasks:3, threads:60} | stream_count: 20 | N/A | N/A |
| └─ExchangeSender_77 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:427ms, min:0s, avg: 142.3ms, p80:427ms, p95:427ms, iters:60, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: test.t1.a, collate: binary], stream_count: 20 | N/A | N/A |
| └─HashJoin_76 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:426ms, min:0s, avg: 142ms, p80:426ms, p95:426ms, iters:60, tasks:3, threads:60} | inner join, equal:[eq(test.t1.a, test.t2.b)] | N/A | N/A |
| ├─ExchangeReceiver_36(Build) | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:374ms, min:0s, avg: 124.7ms, p80:374ms, p95:374ms, iters:46, tasks:3, threads:60} | | N/A | N/A |
| │ └─ExchangeSender_35 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:373.7ms, min:0s, avg: 124.6ms, p80:373.7ms, p95:373.7ms, iters:16, tasks:3, threads:60} | ExchangeType: Broadcast | N/A | N/A |
| │ └─Projection_31 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:373.7ms, min:0s, avg: 124.6ms, p80:373.7ms, p95:373.7ms, iters:16, tasks:3, threads:60} | Column#44, test.t2.b | N/A | N/A |
| │ └─HashAgg_32 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:373.7ms, min:0s, avg: 124.6ms, p80:373.7ms, p95:373.7ms, iters:16, tasks:3, threads:60} | group by:test.t2.b, funcs:sum(Column#45)->Column#44, funcs:firstrow(test.t2.b)->test.t2.b, stream_count: 20 | N/A | N/A |
| │ └─ExchangeReceiver_34 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:373.7ms, min:0s, avg: 124.6ms, p80:373.7ms, p95:373.7ms, iters:16, tasks:3, threads:60} | stream_count: 20 | N/A | N/A |
| │ └─ExchangeSender_33 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:368.3ms, min:0s, avg: 122.8ms, p80:368.3ms, p95:368.3ms, iters:3, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: test.t2.b, collate: binary], stream_count: 20 | N/A | N/A |
| │ └─HashAgg_21 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:367.3ms, min:0s, avg: 122.4ms, p80:367.3ms, p95:367.3ms, iters:3, tasks:3, threads:60} | group by:test.t2.b, funcs:count(1)->Column#45 | N/A | N/A |
| │ └─Selection_30 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:77.2ms, min:0s, avg: 25.7ms, p80:77.2ms, p95:77.2ms, iters:9389, tasks:3, threads:60} | not(isnull(test.t2.b)) | N/A | N/A |
| │ └─TableFullScan_29 | 600000000.00 | 600000000 | mpp[tiflash] | table:t2 | tiflash_task:{proc max:37.2ms, min:0s, avg: 12.4ms, p80:37.2ms, p95:37.2ms, iters:9389, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73834, total_skipped_packs:412, total_scanned_rows:600002896, total_skipped_rows:3342145, total_rs_index_load_time: 0ms, total_read_time: 45ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
| └─Selection_19(Probe) | 100450000.00 | 99980000 | mpp[tiflash] | | tiflash_task:{proc max:30ms, min:0s, avg: 10ms, p80:30ms, p95:30ms, iters:1645, tasks:3, threads:60} | not(isnull(test.t1.a)) | N/A | N/A |
| └─TableFullScan_18 | 100450000.00 | 99980000 | mpp[tiflash] | table:t1 | tiflash_task:{proc max:22ms, min:0s, avg: 7.34ms, p80:22ms, p95:22ms, iters:1645, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:12326, total_skipped_packs:4762, total_scanned_rows:100045535, total_skipped_rows:38651216, total_rs_index_load_time: 4ms, total_read_time: 355ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+------------------------------------------------------+--------------+-----------+--------------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------+--------+------+
18 rows in set (0.46 sec)
Enable Distinct optimization
TiFlash does not support some aggregate functions that accept the Distinct column, such as Sum. By default, the entire aggregate function is calculated in TiDB. By enabling the Distinct optimization, some operations can be pushed down to TiFlash, thereby improving query performance.
If the aggregate function with the distinct operation is slow in a query, you can enable the optimization operation of pushing down the aggregate function with Distinct (such as select sum(distinct a) from t) to Coprocessor by setting the value of the tidb_opt_distinct_agg_push_down variable to ON.
set @@tidb_opt_distinct_agg_push_down = ON;
The following example shows the query result before and after the tidb_opt_distinct_agg_push_down variable is enabled. Before this variable is enabled, TiDB needs to read all data from TiFlash and execute distinct in TiDB. After this variable is enabled, distinct a is pushed down to TiFlash, and a new group by column test.t.a is added in HashAgg_6. The two warnings in the query result indicate that the aggregate function cannot be fully pushed down to TiFlash.
Before tidb_opt_distinct_agg_push_down is enabled:
mysql> explain analyze select count(distinct a) from test.t;
+----------------------------+--------------+-----------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------------+----------+---------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+----------------------------+--------------+-----------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------------+----------+---------+
| HashAgg_6 | 1.00 | 1 | root | | time:2m23.2s, loops:2 | funcs:sum(distinct Column#23)->Column#22 | 41.3 KB | 0 Bytes |
| └─Projection_16 | 600000000.00 | 600000000 | root | | time:2.51s, loops:586548, Concurrency:5 | cast(test.t.a, decimal(10,0) BINARY)->Column#23 | 243.2 KB | N/A |
| └─TableReader_11 | 600000000.00 | 600000000 | root | | time:1.6s, loops:586548, cop_task: {num: 1173, max: 256.2ms, min: 25.1ms, avg: 46.9ms, p95: 63.5ms, rpc_num: 1173, rpc_time: 55s, copr_cache_hit_ratio: 0.00, distsql_concurrency: 15} | data:TableFullScan_10 | 70.2 MB | N/A |
| └─TableFullScan_10 | 600000000.00 | 600000000 | cop[tiflash] | table:t | tiflash_task:{proc max:9ms, min:531µs, avg: 4.49ms, p80:5.55ms, p95:6.74ms, iters:9390, tasks:1173, threads:1173}, tiflash_scan:{dtfile:{total_scanned_packs:73834, total_skipped_packs:1231, total_scanned_rows:600010914, total_skipped_rows:9988978, total_rs_index_load_time: 0ms, total_read_time: 16ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+----------------------------+--------------+-----------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------------+----------+---------+
4 rows in set, 2 warnings (2 min 23.21 sec)
Enable tidb_opt_distinct_agg_push_down:
mysql> set @@tidb_opt_distinct_agg_push_down = ON;
Query OK, 0 rows affected (0.00 sec)
After tidb_opt_distinct_agg_push_down is enabled:
mysql> explain analyze select count(distinct a) from test.t;
+-----------------------------+--------------+-----------+-------------------+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------------+-----------+---------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+-----------------------------+--------------+-----------+-------------------+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------------+-----------+---------+
| HashAgg_10 | 1.00 | 1 | root | | time:233.8ms, loops:2 | funcs:sum(distinct Column#23)->Column#22 | 2.42 KB | 0 Bytes |
| └─Projection_12 | 1.00 | 3 | root | | time:233.7ms, loops:2, Concurrency:OFF | cast(test.c.a, decimal(10,0) BINARY)->Column#23 | 380 Bytes | N/A |
| └─TableReader_11 | 1.00 | 3 | root | | time:233.7ms, loops:2, cop_task: {num: 6, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00, distsql_concurrency: 15} | data:HashAgg_6 | 100 Bytes | N/A |
| └─HashAgg_6 | 1.00 | 3 | batchCop[tiflash] | | tiflash_task:{proc max:225.8ms, min:210.7ms, avg: 216.9ms, p80:225.8ms, p95:225.8ms, iters:3, tasks:3, threads:60} | group by:test.t.a, | N/A | N/A |
| └─TableFullScan_9 | 600000000.00 | 600000000 | batchCop[tiflash] | table:t | tiflash_task:{proc max:50.3ms, min:33.7ms, avg: 44.6ms, p80:50.3ms, p95:50.3ms, iters:9387, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73833, total_skipped_packs:475, total_scanned_rows:600000000, total_skipped_rows:3852098, total_rs_index_load_time: 0ms, total_read_time: 84ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+-----------------------------+--------------+-----------+-------------------+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------------+-----------+---------+
5 rows in set, 2 warnings (0.24 sec)
Compact data using the ALTER TABLE ... COMPACT statement
Executing the ALTER TABLE ... COMPACT statement can initiate compaction for a specific table or partition on a TiFlash node. During the compaction, the physical data on the node is rewritten, including cleaning up deleted rows and merging multiple versions of data caused by updates. This helps enhance access performance and reduce disk usage. The following are examples:
ALTER TABLE employees COMPACT TIFLASH REPLICA;
ALTER TABLE employees COMPACT PARTITION pNorth, pEast TIFLASH REPLICA;
Replace Shuffled Hash Join with Broadcast Hash Join
For Join operations with small tables, the Broadcast Hash Join algorithm can avoid transferring large tables, thereby improving the computing performance.
The
tidb_broadcast_join_threshold_sizevariable controls whether to use the Broadcast Hash Join algorithm. If the table size (unit: byte) is smaller than the value of this variable, the Broadcast Hash Join algorithm is used. Otherwise, the Shuffled Hash Join algorithm is used.set @@tidb_broadcast_join_threshold_size = 2000000;The
tidb_broadcast_join_threshold_countvariable also controls whether to use the Broadcast Hash Join algorithm. If the objects of the join operation belong to a subquery, the optimizer cannot estimate the size of the subquery result set. In this situation, the size is determined by the number of rows in the result set. If the estimated number of rows for the subquery is fewer than the value of this variable, the Broadcast Hash Join algorithm is used. Otherwise, the Shuffled Hash Join algorithm is used.set @@tidb_broadcast_join_threshold_count = 100000;
The following example shows the query result before and after tidb_broadcast_join_threshold_size is re-configured. Before the re-configuration, the ExchangeType of ExchangeSender_29 is HashPartition. After the value of this variable changes to 10000000, the ExchangeType of ExchangeSender_29 changes to Broadcast.
Before tidb_broadcast_join_threshold_size is re-configured:
mysql> explain analyze select max(l_shipdate), max(l_commitdate), max(l_receiptdate) from supplier,lineitem where s_suppkey = l_suppkey;
+------------------------------------------+--------------+-----------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+------------------------------------------+--------------+-----------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+------+
| HashAgg_45 | 1.00 | 1 | root | | time:3.8s, loops:2, partial_worker:{wall_time:3.798284809s, concurrency:5, task_num:1, tot_wait:18.99079929s, tot_exec:8.193µs, tot_time:18.990819019s, max:3.798181723s, p95:3.798181723s}, final_worker:{wall_time:0s, concurrency:5, task_num:1, tot_wait:18.991291379s, tot_exec:197.45µs, tot_time:18.991494363s, max:3.798334972s, p95:3.798334972s} | funcs:max(Column#28)->Column#25, funcs:max(Column#29)->Column#26, funcs:max(Column#30)->Column#27 | 17.3 KB | N/A |
| └─TableReader_47 | 1.00 | 3 | root | | time:3.8s, loops:2, cop_task: {num: 6, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_46 | N/A | N/A |
| └─ExchangeSender_46 | 1.00 | 3 | mpp[tiflash] | | tiflash_task:{proc max:3.79s, min:3.78s, avg: 3.78s, p80:3.79s, p95:3.79s, iters:3, tasks:3, threads:60} | ExchangeType: PassThrough | N/A | N/A |
| └─HashAgg_13 | 1.00 | 3 | mpp[tiflash] | | tiflash_task:{proc max:3.79s, min:3.78s, avg: 3.78s, p80:3.79s, p95:3.79s, iters:3, tasks:3, threads:60} | funcs:max(tpch_100.lineitem.l_shipdate)->Column#28, funcs:max(tpch_100.lineitem.l_commitdate)->Column#29, funcs:max(tpch_100.lineitem.l_receiptdate)->Column#30 | N/A | N/A |
| └─HashJoin_44 | 600845438.27 | 600037902 | mpp[tiflash] | | tiflash_task:{proc max:3.17s, min:3.12s, avg: 3.14s, p80:3.17s, p95:3.17s, iters:11143, tasks:3, threads:60} | inner join, equal:[eq(tpch_100.supplier.s_suppkey, tpch_100.lineitem.l_suppkey)], stream_count: 20 | N/A | N/A |
| ├─ExchangeReceiver_30(Build) | 1000000.00 | 1000000 | mpp[tiflash] | | tiflash_task:{proc max:23.3ms, min:21ms, avg: 22.5ms, p80:23.3ms, p95:23.3ms, iters:120, tasks:3, threads:60} | stream_count: 20 | N/A | N/A |
| │ └─ExchangeSender_29 | 1000000.00 | 1000000 | mpp[tiflash] | | tiflash_task:{proc max:22.7ms, min:0s, avg: 7.57ms, p80:22.7ms, p95:22.7ms, iters:16, tasks:3, threads:2} | ExchangeType: HashPartition, Hash Cols: [name: tpch_100.supplier.s_suppkey, collate: binary], stream_count: 20 | N/A | N/A |
| │ └─TableFullScan_28 | 1000000.00 | 1000000 | mpp[tiflash] | table:supplier | tiflash_task:{proc max:9.71ms, min:0s, avg: 3.24ms, p80:9.71ms, p95:9.71ms, iters:16, tasks:3, threads:2}, tiflash_scan:{dtfile:{total_scanned_packs:123, total_skipped_packs:0, total_scanned_rows:1000000, total_skipped_rows:0, total_rs_index_load_time: 0ms, total_read_time: 2ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
| └─ExchangeReceiver_33(Probe) | 600037902.00 | 600037902 | mpp[tiflash] | | tiflash_task:{proc max:564.3ms, min:340ms, avg: 438.5ms, p80:564.3ms, p95:564.3ms, iters:27583, tasks:3, threads:60} | | N/A | N/A |
| └─ExchangeSender_32 | 600037902.00 | 600037902 | mpp[tiflash] | | tiflash_task:{proc max:3.69s, min:0s, avg: 1.23s, p80:3.69s, p95:3.69s, iters:9298, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: tpch_100.lineitem.l_suppkey, collate: binary] | N/A | N/A |
| └─TableFullScan_31 | 600037902.00 | 600037902 | mpp[tiflash] | table:lineitem | tiflash_task:{proc max:62.8ms, min:0s, avg: 20.9ms, p80:62.8ms, p95:62.8ms, iters:9298, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73465, total_skipped_packs:13060, total_scanned_rows:600168663, total_skipped_rows:106699256, total_rs_index_load_time: 16ms, total_read_time: 27979ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+------------------------------------------+--------------+-----------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+------+
11 rows in set (3.83 sec)
Set tidb_broadcast_join_threshold_size to 10000000:
mysql> set @@tidb_broadcast_join_threshold_size = 10000000;
Query OK, 0 rows affected (0.00 sec)
After tidb_broadcast_join_threshold_size is set to 10000000:
mysql> explain analyze select max(l_shipdate), max(l_commitdate), max(l_receiptdate) from supplier,lineitem where s_suppkey = l_suppkey;
+------------------------------------------+--------------+-----------+--------------+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+------------------------------------------+--------------+-----------+--------------+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+------+
| HashAgg_43 | 1.00 | 1 | root | | time:2.75s, loops:2, partial_worker:{wall_time:2.748445779s, concurrency:5, task_num:1, tot_wait:13.74202679s, tot_exec:8.012µs, tot_time:13.742045721s, max:2.748414915s, p95:2.748414915s}, final_worker:{wall_time:2.74848039s, concurrency:5, task_num:1, tot_wait:13.742157526s, tot_exec:53.191µs, tot_time:13.742214417s, max:2.748462393s, p95:2.748462393s} | funcs:max(Column#28)->Column#25, funcs:max(Column#29)->Column#26, funcs:max(Column#30)->Column#27 | 17.3 KB | N/A |
| └─TableReader_45 | 1.00 | 3 | root | | time:2.75s, loops:2, cop_task: {num: 6, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_44 | N/A | N/A |
| └─ExchangeSender_44 | 1.00 | 3 | mpp[tiflash] | | tiflash_task:{proc max:2.74s, min:2.65s, avg: 2.7s, p80:2.74s, p95:2.74s, iters:3, tasks:3, threads:60} | ExchangeType: PassThrough | N/A | N/A |
| └─HashAgg_13 | 1.00 | 3 | mpp[tiflash] | | tiflash_task:{proc max:2.74s, min:2.65s, avg: 2.7s, p80:2.74s, p95:2.74s, iters:3, tasks:3, threads:60} | funcs:max(tpch_100.lineitem.l_shipdate)->Column#28, funcs:max(tpch_100.lineitem.l_commitdate)->Column#29, funcs:max(tpch_100.lineitem.l_receiptdate)->Column#30 | N/A | N/A |
| └─HashJoin_42 | 600845438.27 | 600037902 | mpp[tiflash] | | tiflash_task:{proc max:2.15s, min:2.06s, avg: 2.11s, p80:2.15s, p95:2.15s, iters:9286, tasks:3, threads:60} | inner join, equal:[eq(tpch_100.supplier.s_suppkey, tpch_100.lineitem.l_suppkey)] | N/A | N/A |
| ├─ExchangeReceiver_30(Build) | 1000000.00 | 3000000 | mpp[tiflash] | | tiflash_task:{proc max:28.1ms, min:24.9ms, avg: 27ms, p80:28.1ms, p95:28.1ms, iters:48, tasks:3, threads:60} | | N/A | N/A |
| │ └─ExchangeSender_29 | 1000000.00 | 1000000 | mpp[tiflash] | | tiflash_task:{proc max:14ms, min:0s, avg: 4.67ms, p80:14ms, p95:14ms, iters:16, tasks:3, threads:2} | ExchangeType: Broadcast | N/A | N/A |
| │ └─TableFullScan_28 | 1000000.00 | 1000000 | mpp[tiflash] | table:supplier | tiflash_task:{proc max:9ms, min:0s, avg: 3ms, p80:9ms, p95:9ms, iters:16, tasks:3, threads:2}, tiflash_scan:{dtfile:{total_scanned_packs:123, total_skipped_packs:0, total_scanned_rows:1000000, total_skipped_rows:0, total_rs_index_load_time: 0ms, total_read_time: 2ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
| └─TableFullScan_31(Probe) | 600037902.00 | 600037902 | mpp[tiflash] | table:lineitem | tiflash_task:{proc max:57.9ms, min:42.9ms, avg: 51.3ms, p80:57.9ms, p95:57.9ms, iters:9297, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73464, total_skipped_packs:12985, total_scanned_rows:600169085, total_skipped_rows:106014866, total_rs_index_load_time: 23ms, total_read_time: 21667ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+------------------------------------------+--------------+-----------+--------------+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+------+
9 rows in set (2.76 sec)
Set a greater execution concurrency
A greater execution concurrency allows TiFlash to occupy more CPU resources of the system, thereby improving query performance.
The tidb_max_tiflash_threads variable is used to set the maximum concurrency for TiFlash to execute a request. The unit is threads.
set @@tidb_max_tiflash_threads = 20;
The following example shows the query result before and after tidb_max_tiflash_threads is re-configured. Before tidb_max_tiflash_threads is set, the concurrency of request execution for a single TiFlash instance is 8 threads. Since the cluster has a total of 3 TiFlash instances, the total number of threads for request execution on all TiFlash instances is 24 (8 × 3). After tidb_max_tiflash_threads is set to 20, the total number of threads for request execution on all TiFlash instances is 60 (20 × 3).
Before tidb_max_tiflash_threads is re-configured:
mysql> explain analyze select a, count(*) from t group by a;
+--------------------------------------+--------------+-----------+--------------+---------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------+--------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+--------------------------------------+--------------+-----------+--------------+---------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------+--------+------+
| TableReader_44 | 20.00 | 20 | root | | time:655.9ms, loops:2, cop_task: {num: 14, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_43 | N/A | N/A |
| └─ExchangeSender_43 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:654.6ms, min:652.4ms, avg: 653.8ms, p80:654.6ms, p95:654.6ms, iters:11, tasks:3, threads:24} | ExchangeType: PassThrough | N/A | N/A |
| └─Projection_5 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:654.3ms, min:651.4ms, avg: 653.1ms, p80:654.3ms, p95:654.3ms, iters:11, tasks:3, threads:24} | test.t.a, Column#22 | N/A | N/A |
| └─Projection_39 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:654.3ms, min:651.4ms, avg: 653.1ms, p80:654.3ms, p95:654.3ms, iters:11, tasks:3, threads:24} | Column#22, test.t.a | N/A | N/A |
| └─HashAgg_40 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:654.3ms, min:651.4ms, avg: 653.1ms, p80:654.3ms, p95:654.3ms, iters:11, tasks:3, threads:24} | group by:test.t.a, funcs:sum(Column#29)->Column#22, funcs:firstrow(test.t.a)->test.t.a, stream_count: 8 | N/A | N/A |
| └─ExchangeReceiver_42 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:654.3ms, min:651.4ms, avg: 653.1ms, p80:654.3ms, p95:654.3ms, iters:11, tasks:3, threads:24} | stream_count: 8 | N/A | N/A |
| └─ExchangeSender_41 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:649ms, min:0s, avg: 216.3ms, p80:649ms, p95:649ms, iters:3, tasks:3, threads:24} | ExchangeType: HashPartition, Hash Cols: [name: test.t.a, collate: binary], stream_count: 8 | N/A | N/A |
| └─HashAgg_37 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:649ms, min:0s, avg: 216.3ms, p80:649ms, p95:649ms, iters:3, tasks:3, threads:24} | group by:test.t.a, funcs:count(1)->Column#29 | N/A | N/A |
| └─TableFullScan_26 | 600000000.00 | 600000000 | mpp[tiflash] | table:t | tiflash_task:{proc max:40ms, min:0s, avg: 13.3ms, p80:40ms, p95:40ms, iters:9386, tasks:3, threads:24}, tiflash_scan:{dtfile:{total_scanned_packs:73833, total_skipped_packs:174, total_scanned_rows:600000000, total_skipped_rows:1402537, total_rs_index_load_time: 0ms, total_read_time: 4ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+--------------------------------------+--------------+-----------+--------------+---------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------+--------+------+
9 rows in set (0.67 sec)
Set tidb_max_tiflash_threads to 20:
mysql> set @@tidb_max_tiflash_threads = 20;
Query OK, 0 rows affected (0.00 sec)
After tidb_max_tiflash_threads is set to 20:
mysql> explain analyze select a, count(*) from t group by a;
+--------------------------------------+--------------+-----------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------+--------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+--------------------------------------+--------------+-----------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------+--------+------+
| TableReader_44 | 20.00 | 20 | root | | time:357.7ms, loops:2, cop_task: {num: 19, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_43 | N/A | N/A |
| └─ExchangeSender_43 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:355.3ms, min:354.6ms, avg: 355ms, p80:355.3ms, p95:355.3ms, iters:16, tasks:3, threads:60} | ExchangeType: PassThrough | N/A | N/A |
| └─Projection_5 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:354.3ms, min:353.6ms, avg: 354ms, p80:354.3ms, p95:354.3ms, iters:16, tasks:3, threads:60} | test.t.a, Column#22 | N/A | N/A |
| └─Projection_39 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:354.3ms, min:353.6ms, avg: 354ms, p80:354.3ms, p95:354.3ms, iters:16, tasks:3, threads:60} | Column#22, test.t.a | N/A | N/A |
| └─HashAgg_40 | 20.00 | 20 | mpp[tiflash] | | tiflash_task:{proc max:354.3ms, min:353.6ms, avg: 354ms, p80:354.3ms, p95:354.3ms, iters:16, tasks:3, threads:60} | group by:test.t.a, funcs:sum(Column#29)->Column#22, funcs:firstrow(test.t.a)->test.t.a, stream_count: 20 | N/A | N/A |
| └─ExchangeReceiver_42 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:354.3ms, min:353.6ms, avg: 354ms, p80:354.3ms, p95:354.3ms, iters:16, tasks:3, threads:60} | stream_count: 20 | N/A | N/A |
| └─ExchangeSender_41 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:349.6ms, min:0s, avg: 116.5ms, p80:349.6ms, p95:349.6ms, iters:3, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: test.t.a, collate: binary], stream_count: 20 | N/A | N/A |
| └─HashAgg_37 | 20.00 | 60 | mpp[tiflash] | | tiflash_task:{proc max:347.6ms, min:0s, avg: 115.9ms, p80:347.6ms, p95:347.6ms, iters:3, tasks:3, threads:60} | group by:test.t.a, funcs:count(1)->Column#29 | N/A | N/A |
| └─TableFullScan_26 | 600000000.00 | 600000000 | mpp[tiflash] | table:t | tiflash_task:{proc max:36.6ms, min:0s, avg: 12.2ms, p80:36.6ms, p95:36.6ms, iters:9389, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73833, total_skipped_packs:418, total_scanned_rows:600001386, total_skipped_rows:3381854, total_rs_index_load_time: 10ms, total_read_time: 61ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+--------------------------------------+--------------+-----------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------+--------+------+
9 rows in set (0.37 sec)
Configure tiflash_fine_grained_shuffle_stream_count
You can increase the concurrency for executing window functions by configuring tiflash_fine_grained_shuffle_stream_count of the Fine Grained Shuffle feature. In this way, the execution of window functions can occupy more system resources, which improves query performance.
When a window function is pushed down to TiFlash for execution, you can use this variable to control the concurrency level of the window function execution. The unit is threads.
set @@tiflash_fine_grained_shuffle_stream_count = 20;
The following example shows the query result before and after the tiflash_fine_grained_shuffle_stream_count variable is re-configured. Before the re-configuration, the stream_count of [ExchangeSender_11, ExchangeReceiver_12, Sort_13, Window_22] is 8. After the re-configuration, the stream_count becomes 20.
Before tiflash_fine_grained_shuffle_stream_count is re-configured:
mysql> explain analyze select *, row_number() over (partition by a) from t;
+----------------------------------+--------------+-----------+--------------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------------------------+--------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+----------------------------------+--------------+-----------+--------------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------------------------+--------+------+
| TableReader_24 | 600000000.00 | 600000000 | root | | time:4m30.5s, loops:585941, cop_task: {num: 9163, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_23 | N/A | N/A |
| └─ExchangeSender_23 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:4m30.5s, min:3m4.8s, avg: 3m36.1s, p80:4m30.5s, p95:4m30.5s, iters:9160, tasks:3, threads:24} | ExchangeType: PassThrough | N/A | N/A |
| └─Window_22 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:3m31.6s, min:2m26.2s, avg: 2m50.7s, p80:3m31.6s, p95:3m31.6s, iters:9160, tasks:3, threads:24} | row_number()->Column#23 over(partition by test.t.a rows between current row and current row), stream_count: 8 | N/A | N/A |
| └─Sort_13 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:3m28.6s, min:2m24.2s, avg: 2m48.4s, p80:3m28.6s, p95:3m28.6s, iters:9160, tasks:3, threads:24} | test.t.a, stream_count: 8 | N/A | N/A |
| └─ExchangeReceiver_12 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:32.4s, min:32s, avg: 32.1s, p80:32.4s, p95:32.4s, iters:49307, tasks:3, threads:24} | stream_count: 8 | N/A | N/A |
| └─ExchangeSender_11 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:32s, min:0s, avg: 10.7s, p80:32s, p95:32s, iters:9386, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: test.t.a, collate: binary], stream_count: 8 | N/A | N/A |
| └─TableFullScan_10 | 600000000.00 | 600000000 | mpp[tiflash] | table:t | tiflash_task:{proc max:113.9ms, min:0s, avg: 38ms, p80:113.9ms, p95:113.9ms, iters:9386, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73834, total_skipped_packs:190, total_scanned_rows:600000000, total_skipped_rows:1536382, total_rs_index_load_time: 16ms, total_read_time: 166324ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+----------------------------------+--------------+-----------+--------------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------------------------------------+--------+------+
7 rows in set (4 min 30.59 sec)
Set tiflash_fine_grained_shuffle_stream_count to 20:
mysql> set @@tiflash_fine_grained_shuffle_stream_count = 20;
Query OK, 0 rows affected (0.00 sec)
After tiflash_fine_grained_shuffle_stream_count is set to 20:
mysql> explain analyze select *, row_number() over (partition by a) from t;
+----------------------------------+--------------+-----------+--------------+---------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------+--------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+----------------------------------+--------------+-----------+--------------+---------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------+--------+------+
| TableReader_24 | 600000000.00 | 600000000 | root | | time:2m55s, loops:585941, cop_task: {num: 9163, max: 0s, min: 0s, avg: 0s, p95: 0s, copr_cache_hit_ratio: 0.00} | data:ExchangeSender_23 | N/A | N/A |
| └─ExchangeSender_23 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:2m55s, min:1m37s, avg: 2m28.7s, p80:2m55s, p95:2m55s, iters:9160, tasks:3, threads:60} | ExchangeType: PassThrough | N/A | N/A |
| └─Window_22 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:2m12.9s, min:1m17s, avg: 1m54.2s, p80:2m12.9s, p95:2m12.9s, iters:9160, tasks:3, threads:60} | row_number()->Column#23 over(partition by test.t.a rows between current row and current row), stream_count: 20 | N/A | N/A |
| └─Sort_13 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:2m10.9s, min:1m16s, avg: 1m52.5s, p80:2m10.9s, p95:2m10.9s, iters:9160, tasks:3, threads:60} | test.t.a, stream_count: 20 | N/A | N/A |
| └─ExchangeReceiver_12 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:27.2s, min:25.5s, avg: 26.6s, p80:27.2s, p95:27.2s, iters:49602, tasks:3, threads:60} | stream_count: 20 | N/A | N/A |
| └─ExchangeSender_11 | 600000000.00 | 600000000 | mpp[tiflash] | | tiflash_task:{proc max:25.5s, min:0s, avg: 8.51s, p80:25.5s, p95:25.5s, iters:9388, tasks:3, threads:60} | ExchangeType: HashPartition, Hash Cols: [name: test.t.a, collate: binary], stream_count: 20 | N/A | N/A |
| └─TableFullScan_10 | 600000000.00 | 600000000 | mpp[tiflash] | table:t | tiflash_task:{proc max:167.3ms, min:0s, avg: 55.8ms, p80:167.3ms, p95:167.3ms, iters:9388, tasks:3, threads:60}, tiflash_scan:{dtfile:{total_scanned_packs:73834, total_skipped_packs:408, total_scanned_rows:600002896, total_skipped_rows:3307316, total_rs_index_load_time: 20ms, total_read_time: 179431ms}, total_create_snapshot_time: 0ms} | keep order:false | N/A | N/A |
+----------------------------------+--------------+-----------+--------------+---------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------+--------+------+
7 rows in set (2 min 55.09 sec)