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make binding doc clearer with examples (pingcap#14879)
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@chrysan @Oreoxmt
chrysan authored and Oreoxmt committed Sep 4, 2023
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Expand Up @@ -120,6 +120,70 @@ SELECT * FROM test . t WHERE a > ?
> ```
>
> 因此包含单个常量的 SQL 语句和包含被逗号连接起来多个常量的 SQL 语句，在被绑定时会被 TiDB 视作不同的 SQL 语句，需要分别创建绑定。
>
> 例如：
>
> ```sql
> CREATE TABLE t(a INT, b INT, KEY idx(a));
> CREATE SESSION BINDING for SELECT * FROM t WHERE a IN (?) USING SELECT /*+ use_index(t, idx) */ * FROM t WHERE a in (?);
> SHOW BINDINGS;
> +-----------------------------------------------+----------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> | Original_sql | Bind_sql | Default_db | Status | Create_time | Update_time | Charset | Collation | Source | Sql_digest | Plan_digest |
> +-----------------------------------------------+----------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> | SELECT * FROM `test` . `t` WHERE `a` IN ( ? ) | SELECT /*+ use_index(`t` `idx`)*/ * FROM `test`.`t` WHERE `a` IN (?) | test | enabled | 2023-08-23 14:15:31.472 | 2023-08-23 14:15:31.472 | utf8mb4 | utf8mb4_general_ci | manual | 8b9c4e6ab8fad5ba29b034311dcbfc8a8ce57dde2e2d5d5b65313b90ebcdebf7 | |
> +-----------------------------------------------+----------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> SELECT * FROM t WHERE a IN (1);
> SELECT @@LAST_PLAN_FROM_BINDING;
> +--------------------------+
> | @@LAST_PLAN_FROM_BINDING |
> +--------------------------+
> | 1 |
> +--------------------------+
> SELECT * FROM t WHERE a IN (1,2);
> SELECT @@LAST_PLAN_FROM_BINDING;
> +--------------------------+
> | @@LAST_PLAN_FROM_BINDING |
> +--------------------------+
> | 0 |
> +--------------------------+
> CREATE SESSION BINDING for SELECT * FROM t WHERE a IN (?,?) USING SELECT /*+ use_index(t, idx) */ * FROM t WHERE a IN (?,?);
> show bindings;
> +-------------------------------------------------+------------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> | Original_sql | Bind_sql | Default_db | Status | Create_time | Update_time | Charset | Collation | Source | Sql_digest | Plan_digest |
> +-------------------------------------------------+------------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> | SELECT * FROM `test` . `t` WHERE `a` IN ( ... ) | SELECT /*+ use_index(`t` `idx`)*/ * FROM `test`.`t` WHERE `a` IN (?,?) | test | enabled | 2023-08-23 14:16:30.762 | 2023-08-23 14:16:30.762 | utf8mb4 | utf8mb4_general_ci | manual | da38bf216db4a53e1a1e01c79ffa42306419442ad7238480bb7ac510723c8bdf | |
> | SELECT * FROM `test` . `t` WHERE `a` IN ( ? ) | SELECT /*+ use_index(`t` `idx`)*/ * FROM `test`.`t` WHERE `a` IN (?) | test | enabled | 2023-08-23 14:15:31.472 | 2023-08-23 14:15:31.472 | utf8mb4 | utf8mb4_general_ci | manual | 8b9c4e6ab8fad5ba29b034311dcbfc8a8ce57dde2e2d5d5b65313b90ebcdebf7 | |
> +-------------------------------------------------+------------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> SELECT * FROM t WHERE a IN (1,2);
> SELECT @@LAST_PLAN_FROM_BINDING;
> +--------------------------+
> | @@LAST_PLAN_FROM_BINDING |
> +--------------------------+
> | 1 |
> +--------------------------+
> SELECT * FROM t WHERE a IN (1,2,3);
> SELECT @@LAST_PLAN_FROM_BINDING;
> +--------------------------+
> | @@LAST_PLAN_FROM_BINDING |
> +--------------------------+
> | 1 |
> +--------------------------+
> DROP SESSION BINDING for SELECT * FROM t WHERE a IN (?);
> SHOW BINDINGS;
> +-------------------------------------------------+------------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> | Original_sql | Bind_sql | Default_db | Status | Create_time | Update_time | Charset | Collation | Source | Sql_digest | Plan_digest |
> +-------------------------------------------------+------------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> | SELECT * FROM `test` . `t` WHERE `a` IN ( ... ) | SELECT /*+ use_index(`t` `idx`)*/ * FROM `test`.`t` WHERE `a` IN (?,?) | test | enabled | 2023-08-23 14:16:30.762 | 2023-08-23 14:16:30.762 | utf8mb4 | utf8mb4_general_ci | manual | da38bf216db4a53e1a1e01c79ffa42306419442ad7238480bb7ac510723c8bdf | |
> +-------------------------------------------------+------------------------------------------------------------------------+------------+---------+-------------------------+-------------------------+---------+--------------------+--------+------------------------------------------------------------------+-------------+
> SELECT * FROM t WHERE a IN (1);
> SELECT @@LAST_PLAN_FROM_BINDING;
> +--------------------------+
> | @@LAST_PLAN_FROM_BINDING |
> +--------------------------+
> | 0 |
> +--------------------------+
> ```
>
值得注意的是，如果一条 SQL 语句在 GLOBAL 和 SESSION 作用域内都有与之绑定的执行计划，因为优化器在遇到 SESSION 绑定时会忽略 GLOBAL 绑定的执行计划，该语句在 SESSION 作用域内绑定的执行计划会屏蔽掉语句在 GLOBAL 作用域内绑定的执行计划。
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