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
Expression Type | Operations |
---|---|
Logical operators | AND (&&), OR (||), NOT (!) |
Comparison functions and operators | <, <=, =, != (<> ), >, >=, <=> , IN() , IS NULL, LIKE, IS TRUE, IS FALSE, COALESCE() |
Numeric functions and operators | +, -, *, /, ABS() , CEIL() , CEILING() , FLOOR() , MOD() |
Control flow functions | CASE , IF() , IFNULL() |
JSON functions | JSON_TYPE(json_val), JSON_EXTRACT(json_doc, path[, path] ...), JSON_OBJECT(key, val[, key, val] ...), JSON_ARRAY([val[, val] ...]), JSON_MERGE(json_doc, json_doc[, json_doc] ...), JSON_SET(json_doc, path, val[, path, val] ...), JSON_INSERT(json_doc, path, val[, path, val] ...), JSON_REPLACE(json_doc, path, val[, path, val] ...), JSON_REMOVE(json_doc, path[, path] ...) |
Date and time functions | DATE_FORMAT() , SYSDATE() |
String functions | RIGHT() |
Blocklist specific expressions
If unexpected behavior occurs in the calculation process when pushing down the supported expressions or specific data types (only the ENUM
type and the BIT
type), you can restore the application quickly by prohibiting the pushdown of the corresponding functions, operators, or data types. Specifically, you can prohibit the functions, operators, or data types from being pushed down by adding them to the blocklist mysql.expr_pushdown_blacklist
. For details, refer to Add to the blocklist.
The schema of mysql.expr_pushdown_blacklist
is as follows:
tidb> 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)
Field description:
name
: the name of the function, operator, or data type that is prohibited from being pushed down.store_type
: specifies to which storage engine the function, operator, or data type is prohibited from being pushed down. Currently, TiDB supports the three storage engines:tikv
,tidb
, andtiflash
.store_type
is case-insensitive. If a function is prohibited from being pushed down to multiple storage engines, use a comma to separate each engine.reason
: The reason why the function is blocklisted.
Add to the blocklist
To add one or more functions, operators, or data types (only the ENUM
type and the BIT
type) to the blocklist, perform the following steps:
Insert the followings to
mysql.expr_pushdown_blacklist
:- the name of the function, operator, or data type to be prohibited from being pushed down
- the storage engine to be prohibited from being pushed down
Execute the
admin reload expr_pushdown_blacklist;
command.
Remove from the blocklist
To remove one or more functions, operators, or data types from the blocklist, perform the following steps:
Delete the name of the function, operator, or data type in
mysql.expr_pushdown_blacklist
.Execute the
admin reload expr_pushdown_blacklist;
command.
Blocklist usage examples
The following example demonstrates how to add the DATE_FORMAT()
function, >
operator, and BIT
data type 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 (0.06 sec)
tidb> 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)
tidb> insert into mysql.expr_pushdown_blacklist values('date_format()', 'tikv',''), ('>','tikv',''), ('bit','tikv','');
Query OK, 2 rows affected (0.01 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 | 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)
tidb> delete from mysql.expr_pushdown_blacklist where name = '>';
Query OK, 1 row affected (0.01 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 | 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)
Operator Name | Aliases |
---|---|
< | lt |
> | gt |
<= | le |
>= | ge |
= | eq |
!= | ne |
<> | ne |
<=> | nulleq |
| | bitor |
&& | bitand |
|| | or |
! | not |
in | in |
+ | plus |
- | minus |
* | mul |
/ | div |
DIV | intdiv |
IS NULL | isnull |
IS TRUE | istrue |
IS FALSE | isfalse |