--- title: "Optimizer Hint - OceanBase 数据库 V4.3.1 | OceanBase 文档中心" description: Optimizer Hint Hint 机制可以使优化器生成指定的执行计划。 一般情况下，优化器会为用户查询选择最佳的执行计划，不需要用户使用 Hint 指定。但在某些场景下，优化器生成的执行计划可能不满足用户的要求，这时就需要用户使用 Hint 来指定生成某种执行计划。 Hint 语法 Hint 从语法上看是一种特… --- 切换语言 - 中文站 - 简体中文 - International - English - 日本站 - 日本語 文档反馈![](https://mdn.alipayobjects.com/huamei_22khvb/afts/img/A*P8CuR4UJ_FkAAAAAAAAAAAAADiGDAQ/original) OceanBase 数据库分布式版 - V 4.3.1 # Optimizer Hint 更新时间：2026-04-10 11:58:01 [编辑](https://github.com/oceanbase/oceanbase-doc/edit/V4.3.1/zh-CN/700.reference/1000.performance-tuning-guide/500.sql-optimization/400.sql-optimization/700.manage-execution-plans/100.optimizer-hint.md) Hint 机制可以使优化器生成指定的执行计划。 一般情况下，优化器会为用户查询选择最佳的执行计划，不需要用户使用 Hint 指定。但在某些场景下，优化器生成的执行计划可能不满足用户的要求，这时就需要用户使用 Hint 来指定生成某种执行计划。 ## Hint 语法 Hint 从语法上看是一种特殊的 SQL 注释，所不同的是在注释的左标记后（"/*"符号）增加了一个"+"。 既然是注释，如果服务器端无法识别 SQL 语句中的 Hint，优化器会选择忽略用户 Hint 而使用默认的计划生成逻辑结构。另外，Hint 只影响优化器生成计划的逻辑结构，而不影响 SQL 语句的语义。 ```sql { DELETE | INSERT | SELECT | UPDATE | REPLACE } /*+ [hint_text][,hint_text]... */ ``` #### 注意 如果使用 MySQL 的 C 客户端执行带 Hint 的 SQL 语句，需要使用 `-c` 选项登录，否则 MySQL 客户端会将 Hint 作为注释从用户 SQL 语句中去除，导致系统无法收到用户 Hint。 ### Hint 参数 Hint 相关参数名称、语义和语法如下表所示。 | **名称** | **语法** | **语义** | | --- | --- | --- | | NO_REWRITE | NO_REWRITE | 禁止 SQL 改写。 | | READ_CONSISTENCY | READ_CONSISTENCY (WEAK[STRONGFROZEN]) | 读一致性设置（弱/强）。 | | INDEX_HINT | /*+ INDEX(table_name index_name) */ | 设置表索引。 | | QUERY_TIMEOUT | QUERY_TIMEOUT(INTNUM) | 设置超时时间。 | | LOG_LEVEL | LOG_LEVEL([']log_level[']) | 设置日志级别，当设置模块级别语句时候，以第一个单引号（'）作为开始，第二个单引号（'）作为结束；例如'DEBUG'。 | | LEADING | LEADING([qb_name] TBL_NAME_LIST) | 设置联接顺序。 | | ORDERED | ORDERED | 设置按照 SQL 中的顺序进行联接。 | | FULL | FULL([qb_name] TBL_NAME) | 设置表访问路径为主表等价于 `INDEX(TBL_NAME PRIMARY)`。 | | USE_PLAN_CACHE | USE_PLAN_CACHE(NONE[DEFAULT]) | 设置是否使用计划缓存： - `NONE`：表示不使用计划缓存。 - `DEFAULT`：表示按照服务器本身的设置来决定是否使用计划缓存。 | | USE_MERGE | USE_MERGE([qb_name] TBL_NAME_LIST) | 设置指定表在作为右表时使用 Merge Join。 | | USE_HASH | USE_HASH([qb_name] TBL_NAME_LIST) | 设置指定表在作为右表时使用 Hash Join。 | | NO_USE_HASH | NO_USE_HASH([qb_name] TBL_NAME_LIST) | 设置指定表在作为右表时不使用 Hash Join。 | | USE_NL | USE_NL([qb_name] TBL_NAME_LIST) | 设置指定表在作为右表时使用 Nested Loop Join。 | | USE_BNL | USE_BNL([qb_name] TBL_NAME_LIST) | 设置指定表在作为右表时使用 Block Nested Loop Join | | USE_HASH_AGGREGATION | USE_HASH_AGGREGATION([qb_name]) | 设置聚合算法为 Hash。例如 Hash Group By 或者 Hash Distinct。 | | NO_USE_HASH_AGGREGATION | NO_USE_HASH_AGGREGATION([qb_name]) | 设置 Aggregate 方法不使用 Hash Aggregate，使用 Merge Group By 或者 Merge Distinct。 | | USE_LATE_MATERIALIZATION | USE_LATE_MATERIALIZATION | 设置使用晚期物化。 | | NO_USE_LATE_MATERIALIZATION | NO_USE_LATE_MATERIALIZATION | 设置不使用晚期物化。 | | TRACE_LOG | TRACE_LOG | 设置收集 Trace 记录用于 `SHOW TRACE` 展示。 | | QB_NAME | QB_NAME( NAME ) | 设置 Query Block 的名称。 | | PARALLEL | PARALLEL(INTNUM) | 设置分布式执行并行度。 | | TOPK | TOPK(PRECISION MINIMUM_ROWS) | 设置模糊查询的精度和最小行数。 其中 `PRECSION` 为整型，取值范围[0，100]，表示模糊查询的行数百分比；`MINIMUM_ROWS` 为最小返回行数。 | | MAX_CONCURRENT\|MAX_CONCURRENT(n) | 限制这个 SQL 文本的并发数。 | #### 说明 - `QB_NAME` 语法是: `@NAME` - `TBL_NAME` 语法是: `[db_name.]relation_name [qb_name]` ## QB_NAME 参数 在 DML 语句中，每一个 `query_block` 都会有一个 `QB_NAME`（Query Block Name），可以由用户指定，也可以由系统自动生成。在用户没有用 Hint 指定 `QB_NAME` 的时候，系统会按照 `SEL$1`、`SEL$2`、`UPD$1`、`DEL$1` 方式从左到右（实际也是 Resolver 的解析顺序）依次生成。 通过 `QB_NAME` 可以精确定位到每一个表，也可以在某处指定任意 Query Block 的行为。`TBL_NAME` 中的 `QB_NAME` 用于定位表，在 Hint 中最前面的 `QB_NAME` 用于定位 Hint 作用于哪一个 `query_block`。 如下例所示，按照默认规则，会为 `SEL$1` 中的 `t1` 表选择 `t1_c1` 路径，为 `SEL$2` 中的 `t2` 表选择主表（Primary）访问。 ```sql obclient> CREATE TABLE t1(c1 INT, c2 INT, KEY t1_c1(c1)); Query OK, 0 rows affected obclient> CREATE TABLE t2(c1 INT, c2 INT, KEY t2_c1(c1)); Query OK, 0 rows affected obclient> EXPLAIN SELECT * FROM t1, (SELECT * FROM t2 WHERE c2 = 1 LIMIT 5) WHERE t1.c1 = 1; +-----------------------------------------------------------------------------------------------------------+ | Query Plan | +-----------------------------------------------------------------------------------------------------------+ | ====================================================================== | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | ---------------------------------------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | |1 |7 | | | |1 |├─TABLE RANGE SCAN |t1(t1_c1) |1 |7 | | | |2 |└─MATERIAL | |1 |3 | | | |3 | └─SUBPLAN SCAN |ANONYMOUS_VIEW1|1 |3 | | | |4 | └─TABLE FULL SCAN |t2 |1 |3 | | | ====================================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t1.c1], [t1.c2], [ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([t1.c1], [t1.c2]), filter(nil), rowset=16 | | access([t1.__pk_increment], [t1.c1], [t1.c2]), partitions(p0) | | is_index_back=true, is_global_index=false, | | range_key([t1.c1], [t1.__pk_increment]), range(1,MIN ; 1,MAX), | | range_cond([t1.c1 = 1]) | | 2 - output([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | 3 - output([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | access([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]) | | 4 - output([t2.c1], [t2.c2]), filter([t2.c2 = 1]), rowset=16 | | access([t2.c2], [t2.c1]), partitions(p0) | | limit(5), offset(nil), is_index_back=false, is_global_index=false, filter_before_indexback[false], | | range_key([t2.__pk_increment]), range(MIN ; MAX)always true | +-----------------------------------------------------------------------------------------------------------+ 25 rows in set ``` 如果 SQL 通过 Hint 来指定 `SEL$1` 的 `t1` 表走主表，`SEL$2` 的 `t2` 表走索引，示例如下： ```sql obclient> EXPLAIN SELECT /*+INDEX(t1 PRIMARY) INDEX(@SEL$2 t2 t2_c1)*/ * FROM t1 , (SELECT * FROM t2 WHERE c2 = 1 LIMIT 5) WHERE t1.c1 = 1; +----------------------------------------------------------------------------------------------------------+ | Query Plan | +----------------------------------------------------------------------------------------------------------+ | ====================================================================== | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | ---------------------------------------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | |1 |3 | | | |1 |├─TABLE FULL SCAN |t1 |1 |3 | | | |2 |└─MATERIAL | |1 |7 | | | |3 | └─SUBPLAN SCAN |ANONYMOUS_VIEW1|1 |7 | | | |4 | └─TABLE FULL SCAN |t2(t2_c1) |1 |7 | | | ====================================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t1.c1], [t1.c2], [ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([t1.c1], [t1.c2]), filter([t1.c1 = 1]), rowset=16 | | access([t1.c1], [t1.c2]), partitions(p0) | | is_index_back=false, is_global_index=false, filter_before_indexback[false], | | range_key([t1.__pk_increment]), range(MIN ; MAX)always true | | 2 - output([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | 3 - output([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | access([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]) | | 4 - output([t2.c1], [t2.c2]), filter([t2.c2 = 1]), rowset=16 | | access([t2.__pk_increment], [t2.c2], [t2.c1]), partitions(p0) | | limit(5), offset(nil), is_index_back=true, is_global_index=false, filter_before_indexback[false], | | range_key([t2.c1], [t2.__pk_increment]), range(MIN,MIN ; MAX,MAX)always true | +----------------------------------------------------------------------------------------------------------+ 24 rows in set ``` #### 说明 由于 `INDEX(t1 PRIMARY)` 本来就存在于 `SEL$1`，可以不用再指定 Hint 作用的 Query Block。 上述示例中 SQL 也可以写成如下方式： ```sql SELECT /*+INDEX(t1 PRIMARY) INDEX(@SEL$2 t2@SEL$2 t2_c1)*/ * FROM t1 , (SELECT * FROM t2 WHERE c2 = 1 LIMIT 5) WHERE t1.c1 = 1; ``` 或： ```sql SELECT /*+INDEX(t1 PRIMARY)*/ * FROM t1 , (SELECT /*+INDEX(t2 t2_c1)*/ * FROM t2 WHERE c2 = 1 LIMIT 5) WHERE t1.c1 = 1; ``` 或： ```sql SELECT /*+INDEX(@SEL$1 t1 PRIMARY) INDEX(@SEL$2 t2 t2_c1)*/ * FROM t1 , (SELECT * FROM t2 WHERE c2 = 1 LIMIT 5) WHERE t1.c1 = 1; ``` 对于该 Hint，可以通过 `EXPLAIN EXTENDED` 命令的执行结果中的 `Outline Data` 来查看它的所有信息。 ```sql obclient> EXPLAIN EXTENDED SELECT * FROM t1, (SELECT * FROM t2 WHERE c2 = 1 LIMIT 5) WHERE t1.c1 = 1; +--------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Query Plan | +--------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ====================================================================== | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | ---------------------------------------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | |1 |7 | | | |1 |├─TABLE RANGE SCAN |t1(t1_c1) |1 |7 | | | |2 |└─MATERIAL | |1 |3 | | | |3 | └─SUBPLAN SCAN |ANONYMOUS_VIEW1|1 |3 | | | |4 | └─TABLE FULL SCAN |t2 |1 |3 | | | ====================================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t1.c1(0x7f20d7035330)], [t1.c2(0x7f20d70358b0)], [ANONYMOUS_VIEW1.c1(0x7f20d7035be0)], [ANONYMOUS_VIEW1.c2(0x7f20d7035f10)]), filter(nil), rowset=16 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([t1.c1(0x7f20d7035330)], [t1.c2(0x7f20d70358b0)]), filter(nil), rowset=16 | | access([t1.__pk_increment(0x7f20d7036b10)], [t1.c1(0x7f20d7035330)], [t1.c2(0x7f20d70358b0)]), partitions(p0) | | is_index_back=true, is_global_index=false, | | range_key([t1.c1(0x7f20d7035330)], [t1.__pk_increment(0x7f20d7036b10)]), range(1,MIN ; 1,MAX), | | range_cond([t1.c1(0x7f20d7035330) = 1(0x7f20d7034b70)]) | | 2 - output([ANONYMOUS_VIEW1.c1(0x7f20d7035be0)], [ANONYMOUS_VIEW1.c2(0x7f20d7035f10)]), filter(nil), rowset=16 | | 3 - output([ANONYMOUS_VIEW1.c1(0x7f20d7035be0)], [ANONYMOUS_VIEW1.c2(0x7f20d7035f10)]), filter(nil), rowset=16 | | access([ANONYMOUS_VIEW1.c1(0x7f20d7035be0)], [ANONYMOUS_VIEW1.c2(0x7f20d7035f10)]) | | 4 - output([t2.c1(0x7f20d7033a10)], [t2.c2(0x7f20d7033490)]), filter([t2.c2(0x7f20d7033490) = 1(0x7f20d7032cd0)]), rowset=16 | | access([t2.c2(0x7f20d7033490)], [t2.c1(0x7f20d7033a10)]), partitions(p0) | | limit(5), offset(nil), is_index_back=false, is_global_index=false, filter_before_indexback[false], | | range_key([t2.__pk_increment(0x7f20d70365e0)]), range(MIN ; MAX)always true | | Used Hint: | | ------------------------------------- | | /*+ | | | | */ | | Qb name trace: | | ------------------------------------- | | stmt_id:0, stmt_type:T_EXPLAIN | | stmt_id:1, SEL$1 | | stmt_id:2, SEL$2 | | Outline Data: | | ------------------------------------- | | /*+ | | BEGIN_OUTLINE_DATA | | LEADING(@"SEL$1" ("aabb"."t1"@"SEL$1" "ANONYMOUS_VIEW1"@"SEL$1")) | | USE_NL(@"SEL$1" "ANONYMOUS_VIEW1"@"SEL$1") | | USE_NL_MATERIALIZATION(@"SEL$1" "ANONYMOUS_VIEW1"@"SEL$1") | | INDEX(@"SEL$1" "aabb"."t1"@"SEL$1" "t1_c1") | | FULL(@"SEL$2" "aabb"."t2"@"SEL$2") | | OPTIMIZER_FEATURES_ENABLE('4.3.0.0') | | END_OUTLINE_DATA | | */ | | Optimization Info: | | ------------------------------------- | | t1: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:1 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t1_c1, t1] | | unstable_index_name:[t1] | | stats version:0 | | dynamic sampling level:0 | | estimation method:[DEFAULT, STORAGE] | | t2: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t2_c1, t2] | | pruned_index_name:[t2_c1] | | stats version:0 | | dynamic sampling level:0 | | estimation method:[DEFAULT, STORAGE] | | Plan Type: | | LOCAL | | Note: | | Degree of Parallelisim is 1 because of table property | +--------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 79 rows in set ``` ## Hint 使用规则 Hint 的一般使用规则如下： - 对于没有指定 Query Block 的 Hint 表明作用于本 Query Block。 **示例 1：由于`t2` 表在 Query Block2，并且无法通过改写提升到 Query Block1，所以 Hint 无法生效。** ```sql obclient> EXPLAIN SELECT /*+INDEX(t2 t2_c1)*/ * FROM t1 , (SELECT * FROM t2 WHERE c2 = 1 LIMIT 5) WHERE t1.c1 = 1; +-----------------------------------------------------------------------------------------------------------+ | Query Plan | +-----------------------------------------------------------------------------------------------------------+ | ====================================================================== | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | ---------------------------------------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | |1 |7 | | | |1 |├─TABLE RANGE SCAN |t1(t1_c1) |1 |7 | | | |2 |└─MATERIAL | |1 |3 | | | |3 | └─SUBPLAN SCAN |ANONYMOUS_VIEW1|1 |3 | | | |4 | └─TABLE FULL SCAN |t2 |1 |3 | | | ====================================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t1.c1], [t1.c2], [ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([t1.c1], [t1.c2]), filter(nil), rowset=16 | | access([t1.__pk_increment], [t1.c1], [t1.c2]), partitions(p0) | | is_index_back=true, is_global_index=false, | | range_key([t1.c1], [t1.__pk_increment]), range(1,MIN ; 1,MAX), | | range_cond([t1.c1 = 1]) | | 2 - output([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | 3 - output([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]), filter(nil), rowset=16 | | access([ANONYMOUS_VIEW1.c1], [ANONYMOUS_VIEW1.c2]) | | 4 - output([t2.c1], [t2.c2]), filter([t2.c2 = 1]), rowset=16 | | access([t2.c2], [t2.c1]), partitions(p0) | | limit(5), offset(nil), is_index_back=false, is_global_index=false, filter_before_indexback[false], | | range_key([t2.__pk_increment]), range(MIN ; MAX)always true | +-----------------------------------------------------------------------------------------------------------+ 25 rows in set ``` **示例 2：如果优化器能够重新整合子查询与外层查询，从而只使用一个 Query Block，那么 Hint 可能生效。** ```sql obclient> EXPLAIN SELECT /*+INDEX(t2 t2_c1)*/ * FROM t1 , (SELECT * FROM t2 WHERE c2 = 1) WHERE t1.c1 = 1; +------------------------------------------------------------------------------------+ | Query Plan | +------------------------------------------------------------------------------------+ | ================================================================ | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | ---------------------------------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | |1 |7 | | | |1 |├─TABLE RANGE SCAN |t1(t1_c1)|1 |7 | | | |2 |└─MATERIAL | |1 |7 | | | |3 | └─TABLE FULL SCAN |t2(t2_c1)|1 |7 | | | ================================================================ | | Outputs & filters: | | ------------------------------------- | | 0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil), rowset=16 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([t1.c1], [t1.c2]), filter(nil), rowset=16 | | access([t1.__pk_increment], [t1.c1], [t1.c2]), partitions(p0) | | is_index_back=true, is_global_index=false, | | range_key([t1.c1], [t1.__pk_increment]), range(1,MIN ; 1,MAX), | | range_cond([t1.c1 = 1]) | | 2 - output([t2.c1], [t2.c2]), filter(nil), rowset=16 | | 3 - output([t2.c2], [t2.c1]), filter([t2.c2 = 1]), rowset=16 | | access([t2.__pk_increment], [t2.c2], [t2.c1]), partitions(p0) | | is_index_back=true, is_global_index=false, filter_before_indexback[false], | | range_key([t2.c1], [t2.__pk_increment]), range(MIN,MIN ; MAX,MAX)always true | +------------------------------------------------------------------------------------+ 22 rows in set ``` 对于上面的示例，如果优化器由于某些原因（比如子查询内部操作阻止了提升合并）无法将子查询 (`SELECT * FROM t2 WHERE c2 = 1`) 合并到外层查询，那么 t2 表仍然被视为子查询的一部分，因此外层查询中的 Hint 无效。 如下示例，由于外层查询和子查询均引用了同一张表 t1，并且都试图利用 PRIMARY 索引，这可能会导致优化器在解释和处理 Hint 时遇到冲突，从而使得 Hint 可能无效。 ```sql obclient> EXPLAIN SELECT /*+INDEX(t1 PRIMARY)*/ * FROM t1 , (SELECT * FROM t1 WHERE c1 = 1) WHERE t1.c1 = 1; +--------------------------------------------------------------------------+ | Query Plan | +--------------------------------------------------------------------------+ | ================================================================ | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | ---------------------------------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | |1 |7 | | | |1 |├─TABLE RANGE SCAN |t1(t1_c1)|1 |7 | | | |2 |└─MATERIAL | |1 |7 | | | |3 | └─TABLE RANGE SCAN |t1(t1_c1)|1 |7 | | | ================================================================ | | Outputs & filters: | | ------------------------------------- | | 0 - output([t1.c1], [t1.c2], [t1.c1], [t1.c2]), filter(nil), rowset=16 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([t1.c1], [t1.c2]), filter(nil), rowset=16 | | access([t1.__pk_increment], [t1.c1], [t1.c2]), partitions(p0) | | is_index_back=true, is_global_index=false, | | range_key([t1.c1], [t1.__pk_increment]), range(1,MIN ; 1,MAX), | | range_cond([t1.c1 = 1]) | | 2 - output([t1.c1], [t1.c2]), filter(nil), rowset=16 | | 3 - output([t1.c1], [t1.c2]), filter(nil), rowset=16 | | access([t1.__pk_increment], [t1.c1], [t1.c2]), partitions(p0) | | is_index_back=true, is_global_index=false, | | range_key([t1.c1], [t1.__pk_increment]), range(1,MIN ; 1,MAX), | | range_cond([t1.c1 = 1]) | +--------------------------------------------------------------------------+ 23 rows in set ``` - 联接方法的 Hint 中指定的表如果找不到，则忽略该表，其他的指定依然生效；如果优化器不能生成指定的联接方法，就会选择其他方法，Hint 无效。 - 联接顺序的 Hint 中如果存在表无法找到的情况，则该 Hint 完全失效。 ## 常用 Hint 及语法 与其他数据库的行为相比，OceanBase 数据库优化器是动态规划的，已经考虑了所有可能的最优路径，Hint 主要作用是指定优化器的行为，并按照 Hint 执行 SQL 查询。 ### INDEX Hint `INDEX` Hint 的语法同时支持 MySQL 模式和 Oracle 模式。 - `INDEX` Hint 的 Oracle 语法如下： ```sql SELECT/*+INDEX(table_name index_name) */ * FROM table_name; ``` - `INDEX` Hint 的 MySQL 语法如下： ```sql table_name [[AS] alias] [index_hint_list] index_hint_list: index_hint [, index_hint] ... index_hint: USE {INDEX|KEY} [FOR {JOIN|ORDER BY|GROUP BY}] ([index_list]) | IGNORE {INDEX|KEY} [FOR {JOIN|ORDER BY|GROUP BY}] (index_list) | FORCE {INDEX|KEY} [FOR {JOIN|ORDER BY|GROUP BY}] (index_list) index_list: index_name [, index_name] ... ``` Oracle 模式的语法中一个表只能指定一个 `INDEX`，而 MySQL 模式的语法可以指定多个。但是 OceanBase 数据库中 MySQL 模式的语法虽然支持指定多个 `INDEX`，但是对于 `USE` 和 `FORCE` 方式，只会用第一个 `INDEX` 生成 `PATH`，即使 SQL 语句中没有该 `INDEX` 的 `filter` 而导致全部扫描同时回表（即 OceanBase 数据库当前设计是认为写 Hint 的人比程序更明白那条路径是更好的）。`IGNORE` 类型会忽略所有指定的 `INDEX`。`USE`、`FORCE` 方式和 Oracle 的Hint 方式实际是一样的，如果该方式的 `INDEX` 不存在或者处于 `invalid` 状态，则 Hint 无效。对于 `IGNORE` 方式，如果将包括主表在内的所有 `INDEX` 忽略，则 Hint 无效。 在 SQL 语句中，表名存在别名即 `table_name [AS] alias`，必须写表别名，才能使 `INDEX` 生效。示例如下： ```sql obclient> create table t1(c1 int, c2 int, c3 int); Query OK, 0 rows affected obclient> create index idx1 on t1(c1); Query OK, 0 rows affected obclient> create index idx2 on t1(c2); Query OK, 0 rows affected obclient> insert into t1 with recursive cte(n) as (select 1 from dual union all select n+1 from cte where n < 1000) select n, mod(n, 3), n from cte; Query OK, 1 row affected obclient> analyze table t1 COMPUTE STATISTICS for all columns size 128; Query OK, 0 rows affected obclient> explain select * from t1 where c1 = 1 and c2 = 1; +-----------------------------------------------------------------------------------+ | Query Plan | +-----------------------------------------------------------------------------------+ | ==================================================== | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | ---------------------------------------------------- | | |0 |TABLE RANGE SCAN|t1(idx1)|1 |7 | | | ==================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t1.c1], [t1.c2], [t1.c3]), filter([t1.c2 = 1]), rowset=16 | | access([t1.__pk_increment], [t1.c1], [t1.c2], [t1.c3]), partitions(p0) | | is_index_back=true, is_global_index=false, filter_before_indexback[false], | | range_key([t1.c1], [t1.__pk_increment]), range(1,MIN ; 1,MAX), | | range_cond([t1.c1 = 1]) | +-----------------------------------------------------------------------------------+ 12 rows in set -------生效的 index obclient> explain select /*+index(t idx2)*/ * from t1 t where c1 = 1 and c2 = 1; +-----------------------------------------------------------------------------------+ | Query Plan | +-----------------------------------------------------------------------------------+ | =================================================== | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | --------------------------------------------------- | | |0 |TABLE RANGE SCAN|t(idx2)|1 |812 | | | =================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t.c1], [t.c2], [t.c3]), filter([t.c1 = 1]), rowset=16 | | access([t.__pk_increment], [t.c1], [t.c2], [t.c3]), partitions(p0) | | is_index_back=true, is_global_index=false, filter_before_indexback[false], | | range_key([t.c2], [t.__pk_increment]), range(1,MIN ; 1,MAX), | | range_cond([t.c2 = 1]) | +-----------------------------------------------------------------------------------+ 12 rows in set -------不生效的 index obclient> explain select /*+index(t1 idx2)*/ * from t1 t where c1 = 1 and c2 = 1; +-----------------------------------------------------------------------------------+ | Query Plan | +-----------------------------------------------------------------------------------+ | =================================================== | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | --------------------------------------------------- | | |0 |TABLE RANGE SCAN|t(idx1)|1 |7 | | | =================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t.c1], [t.c2], [t.c3]), filter([t.c2 = 1]), rowset=16 | | access([t.__pk_increment], [t.c1], [t.c2], [t.c3]), partitions(p0) | | is_index_back=true, is_global_index=false, filter_before_indexback[false], | | range_key([t.c1], [t.__pk_increment]), range(1,MIN ; 1,MAX), | | range_cond([t.c1 = 1]) | +-----------------------------------------------------------------------------------+ 12 rows in set ``` 根据示例结果，`c1 = 1` 过滤性优于 `c2 = 1`，INDEX Hint 不生效时使用优化器索引选择机制使用了 `IDX1`。 ### FULL Hint `FULL` Hint 的语法是用于指定表使用主表扫描，语法如下： ```sql /*+ FULL(table_name)*/ ``` `FULL` Hint 用于为指定表选择主表扫描，等价于 `INDEX` Hint `/*+ INDEX(table_name PRIMARY)*/`。 ### ORDERED Hint `ORDERED` Hint 可以指定按照 `FROM` 后面的表顺序作为联接顺序，语法如下： ```sql /*+ ORDERED */ ``` 如果指定该 Hint 后发生 SQL 改写，那么就按照改写后的 `stmt` 中 From Items 的顺序联接，因为改写时候 `sub_query` 会在 From Items 中对应位置填放新的 Table Item。 ### LEADING Hint `LEADING` Hint 可以指定表的联接顺序，语法如下： ```sql /*+ LEADING(table_name_list)*/ ``` 在 `table_name_list` 中可以使用 `()` 表示内部各表的联接优先级，指定复杂联接。示例如下： ```sql obclient> EXPLAIN BASIC SELECT /*+LEADING(d c b a)*/ * FROM t1 a, t1 b, t1 c, t1 d; +---------------------------------------------------------------------------------------------------------------------------------------+ | Query Plan | +---------------------------------------------------------------------------------------------------------------------------------------+ | ========================================= | | |ID|OPERATOR |NAME| | | ----------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | | | | |1 |├─NESTED-LOOP JOIN CARTESIAN | | | | |2 |│ ├─NESTED-LOOP JOIN CARTESIAN | | | | |3 |│ │ ├─TABLE FULL SCAN |d | | | |4 |│ │ └─MATERIAL | | | | |5 |│ │ └─TABLE FULL SCAN |c | | | |6 |│ └─MATERIAL | | | | |7 |│ └─TABLE FULL SCAN |b | | | |8 |└─MATERIAL | | | | |9 | └─TABLE FULL SCAN |a | | | ========================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([a.c1], [a.c2], [a.c3], [b.c1], [b.c2], [b.c3], [c.c1], [c.c2], [c.c3], [d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([b.c1], [b.c2], [b.c3], [c.c1], [c.c2], [c.c3], [d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 2 - output([c.c1], [c.c2], [c.c3], [d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 3 - output([d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | access([d.c1], [d.c2], [d.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([d.__pk_increment]), range(MIN ; MAX)always true | | 4 - output([c.c1], [c.c2], [c.c3]), filter(nil), rowset=256 | | 5 - output([c.c1], [c.c2], [c.c3]), filter(nil), rowset=256 | | access([c.c1], [c.c2], [c.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([c.__pk_increment]), range(MIN ; MAX)always true | | 6 - output([b.c1], [b.c2], [b.c3]), filter(nil), rowset=256 | | 7 - output([b.c1], [b.c2], [b.c3]), filter(nil), rowset=256 | | access([b.c1], [b.c2], [b.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([b.__pk_increment]), range(MIN ; MAX)always true | | 8 - output([a.c1], [a.c2], [a.c3]), filter(nil), rowset=256 | | 9 - output([a.c1], [a.c2], [a.c3]), filter(nil), rowset=256 | | access([a.c1], [a.c2], [a.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([a.__pk_increment]), range(MIN ; MAX)always true | +---------------------------------------------------------------------------------------------------------------------------------------+ 41 rows in set obclient> EXPLAIN BASIC SELECT /*+LEADING((d c) (b a))*/ * FROM t1 a, t1 b, t1 c, t1 d; +---------------------------------------------------------------------------------------------------------------------------------------+ | Query Plan | +---------------------------------------------------------------------------------------------------------------------------------------+ | ========================================= | | |ID|OPERATOR |NAME| | | ----------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | | | | |1 |├─NESTED-LOOP JOIN CARTESIAN | | | | |2 |│ ├─TABLE FULL SCAN |d | | | |3 |│ └─MATERIAL | | | | |4 |│ └─TABLE FULL SCAN |c | | | |5 |└─MATERIAL | | | | |6 | └─NESTED-LOOP JOIN CARTESIAN | | | | |7 | ├─TABLE FULL SCAN |b | | | |8 | └─MATERIAL | | | | |9 | └─TABLE FULL SCAN |a | | | ========================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([a.c1], [a.c2], [a.c3], [b.c1], [b.c2], [b.c3], [c.c1], [c.c2], [c.c3], [d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([c.c1], [c.c2], [c.c3], [d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 2 - output([d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | access([d.c1], [d.c2], [d.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([d.__pk_increment]), range(MIN ; MAX)always true | | 3 - output([c.c1], [c.c2], [c.c3]), filter(nil), rowset=256 | | 4 - output([c.c1], [c.c2], [c.c3]), filter(nil), rowset=256 | | access([c.c1], [c.c2], [c.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([c.__pk_increment]), range(MIN ; MAX)always true | | 5 - output([a.c1], [a.c2], [a.c3], [b.c1], [b.c2], [b.c3]), filter(nil), rowset=256 | | 6 - output([a.c1], [a.c2], [a.c3], [b.c1], [b.c2], [b.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 7 - output([b.c1], [b.c2], [b.c3]), filter(nil), rowset=256 | | access([b.c1], [b.c2], [b.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([b.__pk_increment]), range(MIN ; MAX)always true | | 8 - output([a.c1], [a.c2], [a.c3]), filter(nil), rowset=256 | | 9 - output([a.c1], [a.c2], [a.c3]), filter(nil), rowset=256 | | access([a.c1], [a.c2], [a.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([a.__pk_increment]), range(MIN ; MAX)always true | +---------------------------------------------------------------------------------------------------------------------------------------+ 41 rows in set obclient> EXPLAIN BASIC SELECT /*+LEADING((d c b) a))*/ * FROM t1 a, t1 b, t1 c, t1 d; +---------------------------------------------------------------------------------------------------------------------------------------+ | Query Plan | +---------------------------------------------------------------------------------------------------------------------------------------+ | ========================================= | | |ID|OPERATOR |NAME| | | ----------------------------------------- | | |0 |NESTED-LOOP JOIN CARTESIAN | | | | |1 |├─NESTED-LOOP JOIN CARTESIAN | | | | |2 |│ ├─NESTED-LOOP JOIN CARTESIAN | | | | |3 |│ │ ├─TABLE FULL SCAN |d | | | |4 |│ │ └─MATERIAL | | | | |5 |│ │ └─TABLE FULL SCAN |c | | | |6 |│ └─MATERIAL | | | | |7 |│ └─TABLE FULL SCAN |b | | | |8 |└─MATERIAL | | | | |9 | └─TABLE FULL SCAN |a | | | ========================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([a.c1], [a.c2], [a.c3], [b.c1], [b.c2], [b.c3], [c.c1], [c.c2], [c.c3], [d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 1 - output([b.c1], [b.c2], [b.c3], [c.c1], [c.c2], [c.c3], [d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 2 - output([c.c1], [c.c2], [c.c3], [d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | conds(nil), nl_params_(nil), use_batch=false | | 3 - output([d.c1], [d.c2], [d.c3]), filter(nil), rowset=256 | | access([d.c1], [d.c2], [d.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([d.__pk_increment]), range(MIN ; MAX)always true | | 4 - output([c.c1], [c.c2], [c.c3]), filter(nil), rowset=256 | | 5 - output([c.c1], [c.c2], [c.c3]), filter(nil), rowset=256 | | access([c.c1], [c.c2], [c.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([c.__pk_increment]), range(MIN ; MAX)always true | | 6 - output([b.c1], [b.c2], [b.c3]), filter(nil), rowset=256 | | 7 - output([b.c1], [b.c2], [b.c3]), filter(nil), rowset=256 | | access([b.c1], [b.c2], [b.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([b.__pk_increment]), range(MIN ; MAX)always true | | 8 - output([a.c1], [a.c2], [a.c3]), filter(nil), rowset=256 | | 9 - output([a.c1], [a.c2], [a.c3]), filter(nil), rowset=256 | | access([a.c1], [a.c2], [a.c3]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([a.__pk_increment]), range(MIN ; MAX)always true | +---------------------------------------------------------------------------------------------------------------------------------------+ 41 rows in set ``` 为确保按照用户指定的顺序进行联接，`LEADING` Hint 的检查比较严格，如果发现 Hint 指定的 `table_name` 不存在，`LEADING` Hint 失效；如果发现 Hint 中存在重复表，`LEADING` Hint 失效。如果在 Optimizer 联接期间，按 `table_id` 无法在 From Items 中找到对应的表，则可能发生改写，那么该表及后面的表指定的 `JOIN` 顺序失效，该表前面的 `JOIN` 顺序依然有效。 ### USE_MERGE Hint `USE_MERGE` 可以指定表在联接时候使用 Merge Join 算法，将所指定的表作为右表。语法如下： ```sql /*+ USE_MERGE(table_name_list) */ ``` #### 注意 OceanBase 数据库中 Merge Join 必须有等值条件的 `join-condition`，因此无等值条件的两个表联接，`use_merge` 会无效。 关于 Merge Join 算法是否认为 A Merge Join B 等效于 B Merge Join A，当前并没有最后结论。按照代价模型，Merge Join 计算代价时是区分左右表的，这也增加了 Hint 的灵活性，所以当前 Merge Join 区分左右表，即 `use_merge` 仅对表作为右表的时候生效。 ### USE_NL Hint 和 join 相关的 Hint 的基本结构如下： `join_hint_name ( @ qb_name table_name_list)` 基本语义为，当联接的右表匹配 table_name_list 时，按照 Hint 语义生成计划。一般需要使用 `LEADING` Hint 指定联接顺序，使 table_name_list 中的表为右表，否则 Hint 会随着联接顺序变化而失效。 其中 table_name_list 可有以下形式： - 单表 use_nl ( t1 ): 以 t1 为右表时使用 Nested Loop Join。 - 多个单表 use_nl ( t1 t2 ... ): 以 t1 或 t2 等为右表时使用 Nested Loop Join。 - 多表 use_nl ( (t1 t2) ): 以 t1 join t2 为右表时使用 Nested Loop Join，忽略 t1/t2 连接顺序和方法。 - 多组表 use_nl ( t1 (t2 t3) (t4 t5 t6) ... ): 以 t1 / t2 join t3 / t4 join t5 join t6 等表为右表时使用 Nested Loop Join。 `USE_NL` 指定表作为右表，在联接的时候使用 Nested Loop Join 算法，语法如下： ```sql /*+ USE_NL(table_name_list) */ ``` 示例如下： ```sql obclient> CREATE TABLE t0(c1 INT, c2 INT, c3 INT); obclient> CREATE TABLE t1(c1 INT, c2 INT, c3 INT); obclient> CREATE TABLE t2(c1 INT, c2 INT, c3 INT); obclient> EXPLAIN EXTENDED SELECT /*+LEADING(t0 t1) USE_NL(t1)*/ * FROM t0, t1 WHERE t0.c1 = t1.c1; +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Query Plan | +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | =================================================== | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | --------------------------------------------------- | | |0 |NESTED-LOOP JOIN | |1 |3 | | | |1 |├─TABLE FULL SCAN |t0 |1 |3 | | | |2 |└─MATERIAL | |1 |3 | | | |3 | └─TABLE FULL SCAN|t1 |1 |3 | | | =================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t0.c1(0x7f218dc21640)], [t0.c2(0x7f218dc21f00)], [t0.c3(0x7f218dc22230)], [t1.c1(0x7f218dc21980)], [t1.c2(0x7f218dc22560)], [t1.c3(0x7f218dc22890)]), filter(nil), rowset=16 | | conds([t0.c1(0x7f218dc21640) = t1.c1(0x7f218dc21980)(0x7f218dc20e80)]), nl_params_(nil), use_batch=false | | 1 - output([t0.c1(0x7f218dc21640)], [t0.c2(0x7f218dc21f00)], [t0.c3(0x7f218dc22230)]), filter(nil), rowset=16 | | access([t0.c1(0x7f218dc21640)], [t0.c2(0x7f218dc21f00)], [t0.c3(0x7f218dc22230)]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([t0.__pk_increment(0x7f218dc230f0)]), range(MIN ; MAX)always true | | 2 - output([t1.c1(0x7f218dc21980)], [t1.c2(0x7f218dc22560)], [t1.c3(0x7f218dc22890)]), filter(nil), rowset=16 | | 3 - output([t1.c1(0x7f218dc21980)], [t1.c2(0x7f218dc22560)], [t1.c3(0x7f218dc22890)]), filter(nil), rowset=16 | | access([t1.c1(0x7f218dc21980)], [t1.c2(0x7f218dc22560)], [t1.c3(0x7f218dc22890)]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([t1.__pk_increment(0x7f218dc23420)]), range(MIN ; MAX)always true | | Used Hint: | | ------------------------------------- | | /*+ | | | | LEADING(("t0" "t1")) | | USE_NL("t1") | | */ | | Qb name trace: | | ------------------------------------- | | stmt_id:0, stmt_type:T_EXPLAIN | | stmt_id:1, SEL$1 | | Outline Data: | | ------------------------------------- | | /*+ | | BEGIN_OUTLINE_DATA | | LEADING(@"SEL$1" ("cccc"."t0"@"SEL$1" "cccc"."t1"@"SEL$1")) | | USE_NL(@"SEL$1" "cccc"."t1"@"SEL$1") | | USE_NL_MATERIALIZATION(@"SEL$1" "cccc"."t1"@"SEL$1") | | FULL(@"SEL$1" "cccc"."t0"@"SEL$1") | | FULL(@"SEL$1" "cccc"."t1"@"SEL$1") | | OPTIMIZER_FEATURES_ENABLE('4.3.0.0') | | END_OUTLINE_DATA | | */ | | Optimization Info: | | ------------------------------------- | | t0: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t0] | | stats version:0 | | dynamic sampling level:1 | | estimation method:[DYNAMIC SAMPLING FULL] | | t1: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t1] | | stats version:0 | | dynamic sampling level:1 | | estimation method:[DYNAMIC SAMPLING FULL] | | Plan Type: | | LOCAL | | Note: | | Degree of Parallelisim is 1 because of table property | +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 74 rows in set obclient> EXPLAIN EXTENDED SELECT /*+LEADING(t0 (t1 t2)) USE_NL((t1 t2))*/ * FROM t0, t1, t2 WHERE t0.c1 = t1.c1 AND t0.c1 = t2.c1; +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Query Plan | +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ===================================================== | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | ----------------------------------------------------- | | |0 |NESTED-LOOP JOIN | |1 |3 | | | |1 |├─TABLE FULL SCAN |t0 |1 |3 | | | |2 |└─MATERIAL | |1 |5 | | | |3 | └─HASH JOIN | |1 |5 | | | |4 | ├─TABLE FULL SCAN|t1 |1 |3 | | | |5 | └─TABLE FULL SCAN|t2 |1 |3 | | | ===================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output([t0.c1(0x7f217d422a30)], [t0.c2(0x7f217d424700)], [t0.c3(0x7f217d424a30)], [t1.c1(0x7f217d422d70)], [t1.c2(0x7f217d424d60)], [t1.c3(0x7f217d425090)], | | [t2.c1(0x7f217d424180)], [t2.c2(0x7f217d4253c0)], [t2.c3(0x7f217d4256f0)]), filter(nil), rowset=16 | | conds([t0.c1(0x7f217d422a30) = t1.c1(0x7f217d422d70)(0x7f217d422270)]), nl_params_(nil), use_batch=false | | 1 - output([t0.c1(0x7f217d422a30)], [t0.c2(0x7f217d424700)], [t0.c3(0x7f217d424a30)]), filter(nil), rowset=16 | | access([t0.c1(0x7f217d422a30)], [t0.c2(0x7f217d424700)], [t0.c3(0x7f217d424a30)]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([t0.__pk_increment(0x7f217d426110)]), range(MIN ; MAX)always true | | 2 - output([t1.c1(0x7f217d422d70)], [t1.c2(0x7f217d424d60)], [t1.c3(0x7f217d425090)], [t2.c1(0x7f217d424180)], [t2.c2(0x7f217d4253c0)], [t2.c3(0x7f217d4256f0)]), filter(nil), rowset=16 | | 3 - output([t1.c1(0x7f217d422d70)], [t1.c2(0x7f217d424d60)], [t1.c3(0x7f217d425090)], [t2.c1(0x7f217d424180)], [t2.c2(0x7f217d4253c0)], [t2.c3(0x7f217d4256f0)]), filter(nil), rowset=16 | | equal_conds([t1.c1(0x7f217d422d70) = t2.c1(0x7f217d424180)(0x7f217d4c2500)]), other_conds(nil) | | 4 - output([t1.c1(0x7f217d422d70)], [t1.c2(0x7f217d424d60)], [t1.c3(0x7f217d425090)]), filter(nil), rowset=16 | | access([t1.c1(0x7f217d422d70)], [t1.c2(0x7f217d424d60)], [t1.c3(0x7f217d425090)]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([t1.__pk_increment(0x7f217d426440)]), range(MIN ; MAX)always true | | 5 - output([t2.c1(0x7f217d424180)], [t2.c2(0x7f217d4253c0)], [t2.c3(0x7f217d4256f0)]), filter(nil), rowset=16 | | access([t2.c1(0x7f217d424180)], [t2.c2(0x7f217d4253c0)], [t2.c3(0x7f217d4256f0)]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([t2.__pk_increment(0x7f217d426770)]), range(MIN ; MAX)always true | | Used Hint: | | ------------------------------------- | | /*+ | | | | LEADING(("t0" ("t1" "t2"))) | | USE_NL(("t1" "t2")) | | */ | | Qb name trace: | | ------------------------------------- | | stmt_id:0, stmt_type:T_EXPLAIN | | stmt_id:1, SEL$1 | | Outline Data: | | ------------------------------------- | | /*+ | | BEGIN_OUTLINE_DATA | | LEADING(@"SEL$1" ("cccc"."t0"@"SEL$1" ("cccc"."t1"@"SEL$1" "cccc"."t2"@"SEL$1"))) | | USE_NL(@"SEL$1" ("cccc"."t2"@"SEL$1" "cccc"."t1"@"SEL$1")) | | USE_NL_MATERIALIZATION(@"SEL$1" ("cccc"."t2"@"SEL$1" "cccc"."t1"@"SEL$1")) | | FULL(@"SEL$1" "cccc"."t0"@"SEL$1") | | USE_HASH(@"SEL$1" "cccc"."t2"@"SEL$1") | | FULL(@"SEL$1" "cccc"."t1"@"SEL$1") | | FULL(@"SEL$1" "cccc"."t2"@"SEL$1") | | OPTIMIZER_FEATURES_ENABLE('4.3.0.0') | | END_OUTLINE_DATA | | */ | | Optimization Info: | | ------------------------------------- | | t0: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t0] | | stats version:0 | | dynamic sampling level:1 | | estimation method:[DYNAMIC SAMPLING FULL] | | t1: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t1] | | stats version:0 | | dynamic sampling level:1 | | estimation method:[DYNAMIC SAMPLING FULL] | | t2: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t2] | | stats version:0 | | dynamic sampling level:1 | | estimation method:[DYNAMIC SAMPLING FULL] | | Plan Type: | | LOCAL | | Note: | | Degree of Parallelisim is 1 because of table property | +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 97 rows in set ``` ### USE_HASH Hint `USE_HASH` 指定表作为右表，在联接的时候使用 Hash Join 算法，语法如下： ```sql /*+ USE_HASH(table_name_list) */ ``` 示例如下： ```sql obclient> CREATE TABLE t0(c1 INT, c2 INT, c3 INT); obclient> CREATE TABLE t1(c1 INT, c2 INT, c3 INT); obclient> EXPLAIN EXTENDED SELECT /*+LEADING(t0 t1) USE_HASH(t1)*/ * FROM t0, t1 WHERE t0.c1 = t1.c1; +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Query Plan | +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ================================================= | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | ------------------------------------------------- | | |0 |HASH JOIN | |1 |5 | | | |1 |├─TABLE FULL SCAN|t0 |1 |3 | | | |2 |└─TABLE FULL SCAN|t1 |1 |3 | | | ================================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([t0.c1(0x7f21a8421640)], [t0.c2(0x7f21a8421f00)], [t0.c3(0x7f21a8422230)], [t1.c1(0x7f21a8421980)], [t1.c2(0x7f21a8422560)], [t1.c3(0x7f21a8422890)]), filter(nil), rowset=16 | | equal_conds([t0.c1(0x7f21a8421640) = t1.c1(0x7f21a8421980)(0x7f21a8420e80)]), other_conds(nil) | | 1 - output([t0.c1(0x7f21a8421640)], [t0.c2(0x7f21a8421f00)], [t0.c3(0x7f21a8422230)]), filter(nil), rowset=16 | | access([t0.c1(0x7f21a8421640)], [t0.c2(0x7f21a8421f00)], [t0.c3(0x7f21a8422230)]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([t0.__pk_increment(0x7f21a84230f0)]), range(MIN ; MAX)always true | | 2 - output([t1.c1(0x7f21a8421980)], [t1.c2(0x7f21a8422560)], [t1.c3(0x7f21a8422890)]), filter(nil), rowset=16 | | access([t1.c1(0x7f21a8421980)], [t1.c2(0x7f21a8422560)], [t1.c3(0x7f21a8422890)]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([t1.__pk_increment(0x7f21a8423420)]), range(MIN ; MAX)always true | | Used Hint: | | ------------------------------------- | | /*+ | | | | LEADING(("t0" "t1")) | | USE_HASH("t1") | | */ | | Qb name trace: | | ------------------------------------- | | stmt_id:0, stmt_type:T_EXPLAIN | | stmt_id:1, SEL$1 | | Outline Data: | | ------------------------------------- | | /*+ | | BEGIN_OUTLINE_DATA | | LEADING(@"SEL$1" ("dddd"."t0"@"SEL$1" "dddd"."t1"@"SEL$1")) | | USE_HASH(@"SEL$1" "dddd"."t1"@"SEL$1") | | FULL(@"SEL$1" "dddd"."t0"@"SEL$1") | | FULL(@"SEL$1" "dddd"."t1"@"SEL$1") | | OPTIMIZER_FEATURES_ENABLE('4.3.0.0') | | END_OUTLINE_DATA | | */ | | Optimization Info: | | ------------------------------------- | | t0: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t0] | | stats version:0 | | dynamic sampling level:1 | | estimation method:[DYNAMIC SAMPLING FULL] | | t1: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[t1] | | stats version:0 | | dynamic sampling level:1 | | estimation method:[DYNAMIC SAMPLING FULL] | | Plan Type: | | LOCAL | | Note: | | Degree of Parallelisim is 1 because of table property | +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 71 rows in set ``` ### PARALLEL Hint `PARALLEL` 指定语句级别的并行度。语法如下： ```sql /*+ PARALLEL(n) */ ``` 其中 `n` 为整数，表示并行度。 OceanBase 数据库也支持表级别的 `PARALLEL` Hint，语法如下： ```sql /*+ PARALLEL(table_name n) */ ``` 如果同时指定了全局并行度和表级并行度，则表级并行度不会生效。示例如下： ```sql obclient> CREATE TABLE tbl1 (col1 INT) PARTITION BY HASH(col1) ; Query OK, 0 rows affected obclient> EXPLAIN SELECT /*+ PARALLEL(3) PARALLEL(tbl1 5) */ * FROM tbl1; +----------------------------------------------------------------------+ | Query Plan | +----------------------------------------------------------------------+ | ========================================================= | | |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| | | --------------------------------------------------------- | | |0 |PX COORDINATOR | |1 |1 | | | |1 |└─EXCHANGE OUT DISTR |:EX10000|1 |1 | | | |2 | └─PX BLOCK ITERATOR| |1 |1 | | | |3 | └─TABLE FULL SCAN|tbl1 |1 |1 | | | ========================================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([INTERNAL_FUNCTION(tbl1.col1)]), filter(nil), rowset=16 | | 1 - output([INTERNAL_FUNCTION(tbl1.col1)]), filter(nil), rowset=16 | | dop=5 | | 2 - output([tbl1.col1]), filter(nil), rowset=16 | | 3 - output([tbl1.col1]), filter(nil), rowset=16 | | access([tbl1.col1]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([tbl1.__pk_increment]), range(MIN ; MAX)always true | +----------------------------------------------------------------------+ 18 rows in set ``` 上一篇 下一篇 ![有帮助](https://gw.alipayobjects.com/mdn/ob_asset/afts/img/A*y6ocSqN8cqsAAAAAAAAAAAAAARQnAQ)![无帮助](https://gw.alipayobjects.com/mdn/ob_asset/afts/img/A*BG9IQJyLHF8AAAAAAAAAAAAAARQnAQ)![反馈](https://gw.alipayobjects.com/mdn/ob_asset/afts/img/A*eTWdQKCRKHwAAAAAAAAAAAAAARQnAQ)[AI](https://www.oceanbase.com/obi) 咨询热线