The Blocklist of Optimization Rules and Expression Pushdown
This document introduces how to use the blocklist of optimization rules and the blocklist of expression pushdown to control the behavior of TiDB.
The blocklist of optimization rules
The blocklist of optimization rules is one way to tune optimization rules, mainly used to manually disable some optimization rules.
Important optimization rules
Optimization Rule | Rule Name | Description |
---|---|---|
Column pruning | column_prune | One operator will prune the column if it is not needed by the upper executor. |
Decorrelate subquery | decorrelate | Tries to rewrite the correlated subquery to non-correlated join or aggregation. |
Aggregation elimination | aggregation_eliminate | Tries to remove unnecessary aggregation operators from the execution plan. |
Projection elimination | projection_eliminate | Removes unnecessary projection operators from the execution plan. |
Max/Min elimination | max_min_eliminate | Rewrites some max/min functions in aggregation to the order by + limit 1 form. |
Predicate pushdown | predicate_push_down | Tries to push predicates down to the operator that is closer to the data source. |
Outer join elimination | outer_join_eliminate | Tries to remove the unnecessary left join or right join from the execution plan. |
Partition pruning | partition_processor | Prunes partitions which are rejected by the predicates and rewrite partitioned table query to the UnionAll + Partition Datasource form. |
Aggregation pushdown | aggregation_push_down | Tries to push aggregations down to their children. |
TopN pushdown | topn_push_down | Tries to push the TopN operator to the place closer to the data source. |
Join reorder | join_reorder | Decides the order of multi-table joins. |
Derive TopN or Limit from window functions | derive_topn_from_window | Derives the TopN or Limit operator from window functions. |
Disable optimization rules
You can use the blocklist of optimization rules to disable some of them if some rules lead to a sub-optimal execution plan for special queries.
Usage
If you want to disable some rules, write its name to the
mysql.opt_rule_blacklist
table. For example:INSERT INTO mysql.opt_rule_blacklist VALUES("join_reorder"), ("topn_push_down");Executing the following SQL statement can make the above operation take effect immediately. The effective range includes all old connections of the corresponding TiDB server:
admin reload opt_rule_blacklist;If you want to re-enable a rule, delete the corresponding data in the table, and then run the
admin reload
statement:DELETE FROM mysql.opt_rule_blacklist WHERE name IN ("join_reorder", "topn_push_down");admin reload opt_rule_blacklist;
The blocklist of expression pushdown
The blocklist of expression pushdown is one way to tune the expression pushdown, mainly used to manually disable some expressions of some specific data types.
Expressions that are supported to be pushed down
For more information about the expressions that are supported to be pushed down, see Supported expressions for pushdown to TiKV.
Disable the pushdown of specific expressions
When you get wrong results due to the expression pushdown, you can use the blocklist to make a quick recovery for the application. More specifically, you can add some of the supported functions or operators to the mysql.expr_pushdown_blacklist
table to disable the pushdown of specific expressions.
The schema of mysql.expr_pushdown_blacklist
is shown as follows:
DESC mysql.expr_pushdown_blacklist;
+------------+--------------+------+------+-------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+--------------+------+------+-------------------+-------+
| name | char(100) | NO | | NULL | |
| store_type | char(100) | NO | | tikv,tiflash,tidb | |
| reason | varchar(200) | YES | | NULL | |
+------------+--------------+------+------+-------------------+-------+
3 rows in set (0.00 sec)
Here is the description of each field above:
name
: The name of the function that is disabled to be pushed down.store_type
: To specify the component that you want to prevent the function from being pushed down to for computing. Available components aretidb
,tikv
, andtiflash
. Thestore_type
is case-insensitive. If you need to specify multiple components, use a comma to separate each component.- When
store_type
istidb
, it indicates whether the function can be executed in other TiDB servers while the TiDB memory table is being read. - When
store_type
istikv
, it indicates whether the function can be executed in TiKV server's Coprocessor component. - When
store_type
istiflash
, it indicates whether the function can be executed in TiFlash Server's Coprocessor component.
- When
reason
: To record the reason why this function is added to the blocklist.
Usage
This section describes how to use the blocklist of expression pushdown.
Add to the blocklist
To add one or more expressions (functions or operators) to the blocklist, perform the following steps:
Insert the corresponding function name or operator name, and the set of components you want to disable the pushdown, to the
mysql.expr_pushdown_blacklist
table.Execute
admin reload expr_pushdown_blacklist
.
Remove from the blocklist
To remove one or more expressions from the blocklist, perform the following steps:
Delete the corresponding function name or operator name, and the set of components you want to disable the pushdown, from the
mysql.expr_pushdown_blacklist
table.Execute
admin reload expr_pushdown_blacklist
.
Expression blocklist usage example
In the following example, the <
and >
operators are added to the blocklist, and then the >
operator is removed from the blocklist.
To judge whether the blocklist takes effect, observe the results of EXPLAIN
(See TiDB Query Execution Plan Overview).
The predicates
a < 2
anda > 2
in theWHERE
clause of the following SQL statement can be pushed down to TiKV.EXPLAIN SELECT * FROM t WHERE a < 2 AND a > 2;+-------------------------+----------+-----------+---------------+------------------------------------+ | id | estRows | task | access object | operator info | +-------------------------+----------+-----------+---------------+------------------------------------+ | TableReader_7 | 0.00 | root | | data:Selection_6 | | └─Selection_6 | 0.00 | cop[tikv] | | gt(ssb_1.t.a, 2), lt(ssb_1.t.a, 2) | | └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo | +-------------------------+----------+-----------+---------------+------------------------------------+ 3 rows in set (0.00 sec)Insert the expression to the
mysql.expr_pushdown_blacklist
table and executeadmin reload expr_pushdown_blacklist
.INSERT INTO mysql.expr_pushdown_blacklist VALUES('<','tikv',''), ('>','tikv','');Query OK, 2 rows affected (0.01 sec) Records: 2 Duplicates: 0 Warnings: 0admin reload expr_pushdown_blacklist;Query OK, 0 rows affected (0.00 sec)Observe the execution plan again and you will find that both the
<
and>
operators are not pushed down to TiKV Coprocessor.EXPLAIN SELECT * FROM t WHERE a < 2 and a > 2;+-------------------------+----------+-----------+---------------+------------------------------------+ | id | estRows | task | access object | operator info | +-------------------------+----------+-----------+---------------+------------------------------------+ | Selection_7 | 10000.00 | root | | gt(ssb_1.t.a, 2), lt(ssb_1.t.a, 2) | | └─TableReader_6 | 10000.00 | root | | data:TableFullScan_5 | | └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo | +-------------------------+----------+-----------+---------------+------------------------------------+ 3 rows in set (0.00 sec)Remove one expression (here is
>
) from the blocklist and executeadmin reload expr_pushdown_blacklist
.DELETE FROM mysql.expr_pushdown_blacklist WHERE name = '>';Query OK, 1 row affected (0.01 sec)admin reload expr_pushdown_blacklist;Query OK, 0 rows affected (0.00 sec)Observe the execution plan again and you will find that
<
is not pushed down while>
is pushed down to TiKV Coprocessor.EXPLAIN SELECT * FROM t WHERE a < 2 AND a > 2;+---------------------------+----------+-----------+---------------+--------------------------------+ | id | estRows | task | access object | operator info | +---------------------------+----------+-----------+---------------+--------------------------------+ | Selection_8 | 0.00 | root | | lt(ssb_1.t.a, 2) | | └─TableReader_7 | 0.00 | root | | data:Selection_6 | | └─Selection_6 | 0.00 | cop[tikv] | | gt(ssb_1.t.a, 2) | | └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo | +---------------------------+----------+-----------+---------------+--------------------------------+ 4 rows in set (0.00 sec)