List of Expressions for Pushdown

When TiDB reads data from TiKV, TiDB tries to push down some expressions (including calculations of functions or operators) to be processed to TiKV. This reduces the amount of transferred data and offloads processing from a single TiDB node. This document introduces the expressions that TiDB already supports pushing down and how to prohibit specific expressions from being pushed down using blocklist.

Supported expressions for pushdown

Blocklist specific expressions

If unexpected behavior occurs during the calculation of a function caused by its pushdown, you can quickly restore the application by blocklisting that function. Specifically, you can prohibit an expression from being pushed down by adding the corresponding functions or operator to the blocklist mysql.expr_pushdown_blacklist.

Add to the blocklist

To add one or more functions or operators to the blocklist, perform the following steps:

1. Insert the function or operator name to mysql.expr_pushdown_blacklist.

2. Execute the admin reload expr_pushdown_blacklist; command.

Remove from the blocklist

To remove one or more functions or operators from the blocklist, perform the following steps:

1. Delete the function or operator name in mysql.expr_pushdown_blacklist.

2. Execute the admin reload expr_pushdown_blacklist; command.

blocklist usage examples

The following example demonstrates how to add the < and > operators to the blocklist, then remove > from the blocklist.

You can see whether the blocklist takes effect by checking the results returned by EXPLAIN statement (See Understanding EXPLAIN results).

tidb> create table t(a int);
Query OK, 0 rows affected (0.01 sec)

tidb> explain select * from t where a < 2 and a > 2;
+---------------------+----------+------+------------------------------------------------------------+
| id                  | count    | task | operator info                                              |
+---------------------+----------+------+------------------------------------------------------------+
| TableReader_7       | 0.00     | root | data:Selection_6                                           |
| └─Selection_6       | 0.00     | cop  | gt(test.t.a, 2), lt(test.t.a, 2)                           |
|   └─TableScan_5     | 10000.00 | cop  | table:t, range:[-inf,+inf], keep order:false, stats:pseudo |
+---------------------+----------+------+------------------------------------------------------------+
3 rows in set (0.00 sec)

tidb> insert into mysql.expr_pushdown_blacklist values('<'), ('>');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

tidb> admin reload expr_pushdown_blacklist;
Query OK, 0 rows affected (0.00 sec)

tidb> explain select * from t where a < 2 and a > 2;
+---------------------+----------+------+------------------------------------------------------------+
| id                  | count    | task | operator info                                              |
+---------------------+----------+------+------------------------------------------------------------+
| Selection_5         | 8000.00  | root | gt(test.t.a, 2), lt(test.t.a, 2)                           |
| └─TableReader_7     | 10000.00 | root | data:TableScan_6                                           |
|   └─TableScan_6     | 10000.00 | cop  | table:t, range:[-inf,+inf], keep order:false, stats:pseudo |
+---------------------+----------+------+------------------------------------------------------------+
3 rows in set (0.00 sec)

tidb> delete from mysql.expr_pushdown_blacklist where name = '>';
Query OK, 1 row affected (0.00 sec)

tidb> admin reload expr_pushdown_blacklist;
Query OK, 0 rows affected (0.00 sec)

tidb> explain select * from t where a < 2 and a > 2;
+-----------------------+----------+------+------------------------------------------------------------+
| id                    | count    | task | operator info                                              |
+-----------------------+----------+------+------------------------------------------------------------+
| Selection_5           | 2666.67  | root | lt(test.t.a, 2)                                            |
| └─TableReader_8       | 3333.33  | root | data:Selection_7                                           |
|   └─Selection_7       | 3333.33  | cop  | gt(test.t.a, 2)                                            |
|     └─TableScan_6     | 10000.00 | cop  | table:t, range:[-inf,+inf], keep order:false, stats:pseudo |
+-----------------------+----------+------+------------------------------------------------------------+
4 rows in set (0.00 sec)