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首页 云原生数据仓库 AnalyticDB PostgreSQL版 开发参考 SQL手册 Hint 通过Hint干预执行计划

通过Hint干预执行计划

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云原生数据仓库AnalyticDB PostgreSQL版支持通过Hint干预执行计划，如指定SQL语句使用或不使用ORCA优化器，干预JOIN顺序和JOIN类型。

前提条件

  • 实例内核小版本为V6.3.7.0及以上版本，如何查看和升级内核小版本，请参见版本升级。

  • 已安装pg_hint_plan插件，具体信息，请参见使用Hint。

指定SQL语句使用或不使用ORCA优化器

  1. 开启Hint干预计划并启用Hint注册规则，语句如下：

    SET pg_hint_plan.enable_hint to on;
SET pg_hint_plan.enable_hint_table to on;

  2. 创建用于测试的表t1和表t2，建表语句如下：

    CREATE TABLE t1 (id int PRIMARY KEY, val int);
CREATE TABLE t2 (id int PRIMARY KEY, val int);

  3. 分别往两张表中插入测试数据，语句如下：

    INSERT INTO t1 SELECT i, i % 100 FROM (SELECT generate_series(1, 10000) i) t;
INSERT INTO t2 SELECT i, i % 10 FROM (SELECT generate_series(1, 1000) i) t;

  4. 创建索引，语句如下：

    CREATE INDEX t1_val ON t1 (val);
CREATE INDEX t2_val ON t2 (val);

  5. 强制关闭ORCA优化器，语句如下：

    SELECT * FROM hint_plan.insert_hint_table($$/*+ SET(optimizer off) */EXPLAIN SELECT * FROM t1 JOIN t2 ON t1.val = t2.val WHERE t1.id = 1;$$);

    返回信息如下：

    -[ RECORD 1 ]---------+-----------------------------------------------------
id                    | 1
norm_query_string     | explain select * from t1 join t2 on t1.val = t2.val;
application_name      |
hints                 | set(optimizer off)
query_hash            | -2169095602568752481
enable                | t
prepare_param_strings | {}

  6. 输出执行计划，语句如下：

    EXPLAIN SELECT * FROM t1 JOIN t2 ON t1.val = t2.val WHERE t1.id = 3;

    强制关闭后，执行计划使用了Postgres query optimizer优化器，返回信息如下：

                                               QUERY PLAN
-------------------------------------------------------------------------------------------------
 Gather Motion 3:1  (slice2; segments: 3)  (cost=8.25..27.61 rows=11 width=16)
   ->  Hash Join  (cost=8.25..27.61 rows=4 width=16)
         Hash Cond: (t2.val = t1.val)
         ->  Seq Scan on t2  (cost=0.00..13.00 rows=334 width=8)
         ->  Hash  (cost=8.22..8.22 rows=1 width=8)
               ->  Broadcast Motion 1:3  (slice1; segments: 1)  (cost=0.16..8.22 rows=3 width=8)
                     ->  Index Scan using t1_pkey on t1  (cost=0.16..8.18 rows=1 width=8)
                           Index Cond: (id = 3)
 Optimizer: Postgres query optimizer
(9 rows)

  7. 强制启用ORCA优化器，语句如下：

    SELECT * FROM hint_plan.insert_hint_table($$/*+ SET(optimizer on) SET(rds_optimizer_options 0) */SELECT * FROM t1 WHERE t1.id = 1;$$);

    返回信息如下：

    -[ RECORD 1 ]---------+------------------------------------------------
id                    | 2
norm_query_string     | select * from t1 where t1.id = $1;
application_name      |
hints                 | set(optimizer on) set(rds_optimizer_options 0)
query_hash            | -8281826471521807124
enable                | t
prepare_param_strings | {}

  8. 输出执行计划，语句如下：

    EXPLAIN SELECT * FROM t1 WHERE t1.id = 2;

    强制启用后，执行计划使用了ORCA优化器，返回信息如下：

                                     QUERY PLAN
----------------------------------------------------------------------------
 Gather Motion 1:1  (slice1; segments: 1)  (cost=0.00..6.00 rows=1 width=8)
   ->  Index Scan using t1_pkey on t1  (cost=0.00..6.00 rows=1 width=8)
         Index Cond: (id = 1)
 Optimizer: Pivotal Optimizer (GPORCA) version 3.86.0
(4 rows)

干预JOIN顺序和JOIN类型

  1. 开启Hint干预计划并启用Hint注册功能，语句如下：

    SET optimizer to off;
SET pg_hint_plan.enable_hint to on;
SET pg_hint_plan.enable_hint_table to on;

  2. 创建用于测试的表t1和表t2，建表语句如下：

    CREATE TABLE t1 (id int PRIMARY KEY, val int);
CREATE TABLE t2 (id int PRIMARY KEY, val int);

  3. 分别往两张表中插入测试数据，语句如下：

    INSERT INTO t1 SELECT i, i % 100 FROM (SELECT generate_series(1, 10000) i) t;
INSERT INTO t2 SELECT i, i % 10 FROM (SELECT generate_series(1, 1000) i) t;

  4. 创建索引，语句如下：

    CREATE INDEX t1_val ON t1 (val);
CREATE INDEX t2_val ON t2 (val);

  5. 输出执行计划，语句如下：

    EXPLAIN SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 100 and t2.val > 1;

    此处为默认执行计划，JOIN类型和JOIN顺序为HashJoin(t1 t2) Leading((t1 t2))，返回信息如下：

                                        QUERY PLAN
----------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=25.50..208.00 rows=800 width=16)
   ->  Hash Join  (cost=25.50..208.00 rows=267 width=16)
         Hash Cond: (t1.id = t2.id)
         ->  Seq Scan on t1  (cost=0.00..137.00 rows=3334 width=8)
               Filter: (val < 100)
         ->  Hash  (cost=15.50..15.50 rows=267 width=8)
               ->  Seq Scan on t2  (cost=0.00..15.50 rows=267 width=8)
                     Filter: (val > 1)
 Optimizer: Postgres query optimizer

  6. 以下几种方法可以干预JOIN顺序和JOIN类型：

    • 在输出执行计划的语句前添加Hint，语句如下：

      /*+ MergeJoin(t1 t2) Leading((t2 t1)) */ EXPLAIN SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 100 and t2.val > 1;

      JOIN类型由HashJoin转变为MergeJoin，JOIN顺序由(t1,t2)转变为(t2,t1)，返回信息如下：

                                          QUERY PLAN
-----------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=0.31..93.54 rows=800 width=16)
   ->  Merge Join  (cost=0.31..93.54 rows=267 width=16)
         Merge Cond: (t2.id = t1.id)
         ->  Index Scan using t2_pkey on t2  (cost=0.15..52.90 rows=267 width=8)
               Filter: (val > 1)
         ->  Index Scan using t1_pkey on t1  (cost=0.16..279.91 rows=3334 width=8)
               Filter: (val < 100)
 Optimizer: Postgres query optimizer
(8 rows)

    • 先注册Hint再输出执行计划：

      1. 注册SQL语句模板，语句如下：

        SELECT * FROM hint_plan.insert_hint_table($$/*+ MergeJoin(t1 t2) Leading((t2 t1)) */SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 100 and t2.val > 1;$$);

        返回信息如下：

        -[ RECORD 1 ]---------+--------------------------------------------------------------------------
id                    | 1
norm_query_string     | SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < $1 and t2.val > $2;
application_name      |
hints                 | MergeJoin(t1 t2) Leading((t2 t1))
query_hash            | -4733464863014584191
enable                | t
prepare_param_strings | {}

      2. 输出执行计划，语句如下：

        EXPLAIN SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 100 and t2.val > 1;

        Hint已生效，JOIN类型由HashJoin转变为MergeJoin，JOIN顺序由(t1,t2)转变为(t2,t1)，返回信息如下：

                                            QUERY PLAN
-----------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=0.31..93.54 rows=800 width=16)
   ->  Merge Join  (cost=0.31..93.54 rows=267 width=16)
         Merge Cond: (t2.id = t1.id)
         ->  Index Scan using t2_pkey on t2  (cost=0.15..52.90 rows=267 width=8)
               Filter: (val > 1)
         ->  Index Scan using t1_pkey on t1  (cost=0.16..279.91 rows=3334 width=8)
               Filter: (val < 100)
 Optimizer: Postgres query optimizer
(8 rows)

      3. 执行相同SQL语句模板的查询计划，语句如下：

        EXPLAIN SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 80 and t2.val > 20;

        返回信息如下，可以看到Hint也可以对同SQL语句模板的其它SQL语句生效：

                                            QUERY PLAN
-----------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=8.25..38.24 rows=13 width=16)
   ->  Merge Join  (cost=8.25..38.24 rows=5 width=16)
         Merge Cond: (t1.id = t2.id)
         ->  Index Scan using t1_pkey on t1  (cost=0.16..279.91 rows=2667 width=8)
               Filter: (val < 80)
         ->  Sort  (cost=8.09..8.09 rows=1 width=8)
               Sort Key: t2.id
               ->  Index Scan using t2_val on t2  (cost=0.15..8.08 rows=1 width=8)
                     Index Cond: (val > 20)
 Optimizer: Postgres query optimizer
(10 rows)

      4. 可选：删除所有该参数化语句的Hint，语句如下：

        SELECT * FROM hint_plan.delete_all_hint_table($$SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 100 and t2.val > 1;$$);

    • 语句中部分常数固定，注册Hint后执行计划：

      1. 使用PREPARE语句注册SQL语句模板，语句如下：

        SELECT * FROM hint_plan.insert_hint_table($$/*+ MergeJoin(t1 t2) Leading((t2 t1)) */PREPARE a AS SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 100 and t2.val > $1;$$);

        返回信息如下：

        -[ RECORD 1 ]---------+--------------------------------------------------------------------------
id                    | 1
norm_query_string     | PREPARE a AS SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < $1 and t2.val > $2;
application_name      |
hints                 | MergeJoin(t1 t2) Leading((t2 t1))
query_hash            | -4733464863014584191
enable                | t
prepare_param_strings | {100,$1}

      2. 执行变更了参数化常数的执行计划，语句如下：

        EXPLAIN SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 100 and t2.val > 2;

        参数化常数变化后，Hint依然可以生效，JOIN类型由HashJoin转变为MergeJoin，JOIN顺序由(t1,t2)转变为(t2,t1)，返回信息如下：

                                            QUERY PLAN
-----------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=0.31..93.54 rows=800 width=16)
   ->  Merge Join  (cost=0.31..93.54 rows=267 width=16)
         Merge Cond: (t1.id = t2.id)
         ->  Index Scan using t1_pkey on t1  (cost=0.16..279.91 rows=3334 width=8)
               Filter: (val < 100)
         ->  Index Scan using t2_pkey on t2  (cost=0.15..52.90 rows=267 width=8)
               Filter: (val > 2)
 Optimizer: Postgres query optimizer
(8 rows)

      3. 执行变更了固定常数的执行计划，语句如下：

        EXPLAIN SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 80 and t2.val > 20;

        固定常数变化后，Hint失效，不再干预执行计划，返回信息如下：

                                            QUERY PLAN
-----------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=8.09..175.22 rows=13 width=16)
   ->  Hash Join  (cost=8.09..175.22 rows=5 width=16)
         Hash Cond: (t1.id = t2.id)
         ->  Seq Scan on t1  (cost=0.00..137.00 rows=2667 width=8)
               Filter: (val < 80)
         ->  Hash  (cost=8.08..8.08 rows=1 width=8)
               ->  Index Scan using t2_val on t2  (cost=0.15..8.08 rows=1 width=8)
                     Index Cond: (val > 20)
 Optimizer: Postgres query optimizer
(9 rows)

      4. 可选：删除所有该参数化语句的Hint，语句如下：

        SELECT * FROM hint_plan.delete_all_hint_table($$SELECT * FROM t1, t2 WHERE t1.id = t2.id and t1.val < 100 and t2.val > 1;$$);
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