CREATE [GLOBAL|SESSION] BINDING
CREATE [GLOBAL|SESSION] BINDING 语句用于在 TiDB 中创建新的执行计划绑定。绑定可用于将优化器 Hint 插入语句中，而无需更改底层查询。
BINDING 语句可以在 GLOBAL 或者 SESSION 作用域内创建执行计划绑定。在不指定作用域时，默认的作用域为 SESSION。
被绑定的 SQL 语句会被参数化后存储到系统表中。在处理 SQL 查询时，只要参数化后的 SQL 语句和系统表中某个被绑定的 SQL 语句一致，并且系统变量 tidb_use_plan_baselines 的值为 ON（其默认值为 ON），即可使用相应的优化器 Hint。如果存在多个可匹配的执行计划，优化器会从中选择代价最小的一个进行绑定。更多信息，请参考创建绑定。
语法图
- CreateBindingStmt
- GlobalScope
- BindableStmt
CreateBindingStmt ::=
'CREATE' GlobalScope 'BINDING' ( 'FOR' BindableStmt 'USING' BindableStmt
| 'FROM' 'HISTORY' 'USING' 'PLAN' 'DIGEST' PlanDigest )
GlobalScope ::=
( 'GLOBAL' | 'SESSION' )?
BindableStmt ::=
( SelectStmt | UpdateStmt | InsertIntoStmt | ReplaceIntoStmt | DeleteStmt )
示例
你可以根据 SQL 或历史执行计划创建绑定。
下面的示例演示如何根据 SQL 创建绑定。
CREATE TABLE t1 (
id INT NOT NULL PRIMARY KEY auto_increment,
b INT NOT NULL,
pad VARBINARY(255),
INDEX(b)
);
Query OK, 0 rows affected (0.07 sec)
INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM dual;
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
Query OK, 8 rows affected (0.00 sec)
Records: 8 Duplicates: 0 Warnings: 0
INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
Query OK, 1000 rows affected (0.04 sec)
Records: 1000 Duplicates: 0 Warnings: 0
INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
Query OK, 100000 rows affected (1.74 sec)
Records: 100000 Duplicates: 0 Warnings: 0
INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
Query OK, 100000 rows affected (2.15 sec)
Records: 100000 Duplicates: 0 Warnings: 0
INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
Query OK, 100000 rows affected (2.64 sec)
Records: 100000 Duplicates: 0 Warnings: 0
SELECT SLEEP(1);
+----------+
| SLEEP(1) |
+----------+
| 0 |
+----------+
1 row in set (1.00 sec)
ANALYZE TABLE t1;
Query OK, 0 rows affected (1.33 sec)
EXPLAIN ANALYZE SELECT * FROM t1 WHERE b = 123;
+-------------------------------+---------+---------+-----------+----------------------+---------------------------------------------------------------------------+-----------------------------------+----------------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+-------------------------------+---------+---------+-----------+----------------------+---------------------------------------------------------------------------+-----------------------------------+----------------+------+
| IndexLookUp_10 | 583.00 | 297 | root | | time:10.545072ms, loops:2, rpc num: 1, rpc time:398.359µs, proc keys:297 | | 109.1484375 KB | N/A |
| ├─IndexRangeScan_8(Build) | 583.00 | 297 | cop[tikv] | table:t1, index:b(b) | time:0s, loops:4 | range:[123,123], keep order:false | N/A | N/A |
| └─TableRowIDScan_9(Probe) | 583.00 | 297 | cop[tikv] | table:t1 | time:12ms, loops:4 | keep order:false | N/A | N/A |
+-------------------------------+---------+---------+-----------+----------------------+---------------------------------------------------------------------------+-----------------------------------+----------------+------+
3 rows in set (0.02 sec)
CREATE SESSION BINDING FOR
SELECT * FROM t1 WHERE b = 123
USING
SELECT * FROM t1 IGNORE INDEX (b) WHERE b = 123;
Query OK, 0 rows affected (0.00 sec)
EXPLAIN ANALYZE SELECT * FROM t1 WHERE b = 123;
+-------------------------+-----------+---------+-----------+---------------+--------------------------------------------------------------------------------+--------------------+---------------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+-------------------------+-----------+---------+-----------+---------------+--------------------------------------------------------------------------------+--------------------+---------------+------+
| TableReader_7 | 583.00 | 297 | root | | time:222.32506ms, loops:2, rpc num: 1, rpc time:222.078952ms, proc keys:301010 | data:Selection_6 | 88.6640625 KB | N/A |
| └─Selection_6 | 583.00 | 297 | cop[tikv] | | time:224ms, loops:298 | eq(test.t1.b, 123) | N/A | N/A |
| └─TableFullScan_5 | 301010.00 | 301010 | cop[tikv] | table:t1 | time:220ms, loops:298 | keep order:false | N/A | N/A |
+-------------------------+-----------+---------+-----------+---------------+--------------------------------------------------------------------------------+--------------------+---------------+------+
3 rows in set (0.22 sec)
SHOW SESSION BINDINGS\G
*************************** 1. row ***************************
Original_sql: select * from t1 where b = ?
Bind_sql: SELECT * FROM t1 IGNORE INDEX (b) WHERE b = 123
Default_db: test
Status: using
Create_time: 2020-05-22 14:38:03.456
Update_time: 2020-05-22 14:38:03.456
Charset: utf8mb4
Collation: utf8mb4_0900_ai_ci
1 row in set (0.00 sec)
DROP SESSION BINDING FOR SELECT * FROM t1 WHERE b = 123;
Query OK, 0 rows affected (0.00 sec)
EXPLAIN ANALYZE SELECT * FROM t1 WHERE b = 123;
+-------------------------------+---------+---------+-----------+----------------------+-------------------------------------------------------------------------+-----------------------------------+----------------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+-------------------------------+---------+---------+-----------+----------------------+-------------------------------------------------------------------------+-----------------------------------+----------------+------+
| IndexLookUp_10 | 583.00 | 297 | root | | time:5.31206ms, loops:2, rpc num: 1, rpc time:665.927µs, proc keys:297 | | 109.1484375 KB | N/A |
| ├─IndexRangeScan_8(Build) | 583.00 | 297 | cop[tikv] | table:t1, index:b(b) | time:0s, loops:4 | range:[123,123], keep order:false | N/A | N/A |
| └─TableRowIDScan_9(Probe) | 583.00 | 297 | cop[tikv] | table:t1 | time:0s, loops:4 | keep order:false | N/A | N/A |
+-------------------------------+---------+---------+-----------+----------------------+-------------------------------------------------------------------------+-----------------------------------+----------------+------+
3 rows in set (0.01 sec)
下面的示例演示如何根据历史执行计划创建绑定。
mysql> CREATE TABLE t(id INT PRIMARY KEY , a INT, KEY(a));
Query OK, 0 rows affected (0.06 sec)
mysql> SELECT /*+ IGNORE_INDEX(t, a) */ * FROM t WHERE a = 1;
Empty set (0.01 sec)
mysql> SELECT plan_digest FROM INFORMATION_SCHEMA.STATEMENTS_SUMMARY WHERE QUERY_SAMPLE_TEXT = 'SELECT /*+ IGNORE_INDEX(t, a) */ * FROM t WHERE a = 1';
+------------------------------------------------------------------+
| plan_digest |
+------------------------------------------------------------------+
| 4e3159169cc63c14b139a4e7d72eae1759875c9a9581f94bb2079aae961189cb |
+------------------------------------------------------------------+
1 row in set (0.01 sec)
mysql> CREATE BINDING FROM HISTORY USING PLAN DIGEST '4e3159169cc63c14b139a4e7d72eae1759875c9a9581f94bb2079aae961189cb';
Query OK, 0 rows affected (0.02 sec)
mysql> SELECT * FROM t WHERE a = 1;
Empty set (0.01 sec)
mysql> SELECT @@LAST_PLAN_FROM_BINDING;
+--------------------------+
| @@LAST_PLAN_FROM_BINDING |
+--------------------------+
| 1 |
+--------------------------+
1 row in set (0.01 sec)
MySQL 兼容性
CREATE [GLOBAL|SESSION] BINDING 语句是 TiDB 对 MySQL 语法的扩展。