sql - 分区表查询仍在扫描所有分区

Question

我有一个包含超过 10 亿条记录的表。为了提高性能，我把它分成了30个分区。最频繁的查询在它们的 where 子句中有 (id = ...)，所以我决定在 id 列上对表进行分区。

基本上，分区是这样创建的:

CREATE TABLE foo_0 (CHECK (id % 30 = 0)) INHERITS (foo);
CREATE TABLE foo_1 (CHECK (id % 30 = 1)) INHERITS (foo);
CREATE TABLE foo_2 (CHECK (id % 30 = 2)) INHERITS (foo);
CREATE TABLE foo_3 (CHECK (id % 30 = 3)) INHERITS (foo);
.
.
.

我为整个数据库运行了 ANALYZE，特别是，我通过运行让它为这个表的 id 列收集额外的统计信息:

ALTER TABLE foo ALTER COLUMN id SET STATISTICS 10000;

但是，当我运行在 id 列上过滤的查询时，规划器显示它仍在扫描所有分区。 constraint_exclusion 设置为 partition，所以这不是问题所在。

EXPLAIN ANALYZE SELECT * FROM foo WHERE (id = 2);


                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------
 Result  (cost=0.00..8106617.40 rows=3620981 width=54) (actual time=30.544..215.540 rows=171477 loops=1)
   ->  Append  (cost=0.00..8106617.40 rows=3620981 width=54) (actual time=30.539..106.446 rows=171477 loops=1)
         ->  Seq Scan on foo  (cost=0.00..0.00 rows=1 width=203) (actual time=0.002..0.002 rows=0 loops=1)
               Filter: (id = 2)
         ->  Bitmap Heap Scan on foo_0 foo  (cost=3293.44..281055.75 rows=122479 width=52) (actual time=0.020..0.020 rows=0 loops=1)
               Recheck Cond: (id = 2)
               ->  Bitmap Index Scan on foo_0_idx_1  (cost=0.00..3262.82 rows=122479 width=0) (actual time=0.018..0.018 rows=0 loops=1)
                     Index Cond: (id = 2)
         ->  Bitmap Heap Scan on foo_1 foo  (cost=3312.59..274769.09 rows=122968 width=56) (actual time=0.012..0.012 rows=0 loops=1)
               Recheck Cond: (id = 2)
               ->  Bitmap Index Scan on foo_1_idx_1  (cost=0.00..3281.85 rows=122968 width=0) (actual time=0.010..0.010 rows=0 loops=1)
                     Index Cond: (id = 2)
         ->  Bitmap Heap Scan on foo_2 foo  (cost=3280.30..272541.10 rows=121903 width=56) (actual time=30.504..77.033 rows=171477 loops=1)
               Recheck Cond: (id = 2)
               ->  Bitmap Index Scan on foo_2_idx_1  (cost=0.00..3249.82 rows=121903 width=0) (actual time=29.825..29.825 rows=171477 loops=1)
                     Index Cond: (id = 2)
.
.
.

我该怎么做才能让刨床有更好的计划？我是否还需要为所有分区运行 ALTER TABLE foo ALTER COLUMN id SET STATISTICS 10000;？

编辑

在使用 Erwin 对查询的建议更改后，规划器仅扫描正确的分区，但执行时间实际上比完整扫描(至少是索引)更差。

EXPLAIN ANALYZE select * from foo where (id % 30 = 2) and (id = 2);
                                                                         QUERY PLAN
                                                                             QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Result  (cost=0.00..8106617.40 rows=3620981 width=54) (actual time=32.611..224.934 rows=171477 loops=1)
   ->  Append  (cost=0.00..8106617.40 rows=3620981 width=54) (actual time=32.606..116.565 rows=171477 loops=1)
         ->  Seq Scan on foo  (cost=0.00..0.00 rows=1 width=203) (actual time=0.002..0.002 rows=0 loops=1)
               Filter: (id = 2)
         ->  Bitmap Heap Scan on foo_0 foo  (cost=3293.44..281055.75 rows=122479 width=52) (actual time=0.046..0.046 rows=0 loops=1)
               Recheck Cond: (id = 2)
               ->  Bitmap Index Scan on foo_0_idx_1  (cost=0.00..3262.82 rows=122479 width=0) (actual time=0.044..0.044 rows=0 loops=1)
                     Index Cond: (id = 2)
         ->  Bitmap Heap Scan on foo_1 foo  (cost=3312.59..274769.09 rows=122968 width=56) (actual time=0.021..0.021 rows=0 loops=1)
               Recheck Cond: (id = 2)
               ->  Bitmap Index Scan on foo_1_idx_1  (cost=0.00..3281.85 rows=122968 width=0) (actual time=0.020..0.020 rows=0 loops=1)
                     Index Cond: (id = 2)
         ->  Bitmap Heap Scan on foo_2 foo  (cost=3280.30..272541.10 rows=121903 width=56) (actual time=32.536..86.730 rows=171477 loops=1)
               Recheck Cond: (id = 2)
               ->  Bitmap Index Scan on foo_2_idx_1  (cost=0.00..3249.82 rows=121903 width=0) (actual time=31.842..31.842 rows=171477 loops=1)
                     Index Cond: (id = 2)
         ->  Bitmap Heap Scan on foo_3 foo  (cost=3475.87..285574.05 rows=129032 width=52) (actual time=0.035..0.035 rows=0 loops=1)
               Recheck Cond: (id = 2)
               ->  Bitmap Index Scan on foo_3_idx_1  (cost=0.00..3443.61 rows=129032 width=0) (actual time=0.031..0.031 rows=0 loops=1)
.
.
.
         ->  Bitmap Heap Scan on foo_29 foo  (cost=3401.84..276569.90 rows=126245 width=56) (actual time=0.019..0.019 rows=0 loops=1)
               Recheck Cond: (id = 2)
               ->  Bitmap Index Scan on foo_29_idx_1  (cost=0.00..3370.28 rows=126245 width=0) (actual time=0.018..0.018 rows=0 loops=1)
                     Index Cond: (id = 2)
 Total runtime: 238.790 ms

对比:

EXPLAIN ANALYZE select * from foo where (id % 30 = 2) and (id = 2);
                                                                            QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Result  (cost=0.00..273120.30 rows=611 width=56) (actual time=31.519..257.051 rows=171477 loops=1)
   ->  Append  (cost=0.00..273120.30 rows=611 width=56) (actual time=31.516..153.356 rows=171477 loops=1)
         ->  Seq Scan on foo  (cost=0.00..0.00 rows=1 width=203) (actual time=0.002..0.002 rows=0 loops=1)
               Filter: ((id = 2) AND ((id % 30) = 2))
         ->  Bitmap Heap Scan on foo_2 foo  (cost=3249.97..273120.30 rows=610 width=56) (actual time=31.512..124.177 rows=171477 loops=1)
               Recheck Cond: (id = 2)
               Filter: ((id % 30) = 2)
               ->  Bitmap Index Scan on foo_2_idx_1  (cost=0.00..3249.82 rows=121903 width=0) (actual time=30.816..30.816 rows=171477 loops=1)
                     Index Cond: (id = 2)
 Total runtime: 270.384 ms

Answer 1

对于重要的表达式，您必须在查询中重复或多或少的逐字条件，以使 Postgres 查询计划器理解它可以依赖 CHECK 约束。即使它看起来多余!

Per documentation :

With constraint exclusion enabled, the planner will examine the constraints of each partition and try to prove that the partition need not be scanned because it could not contain any rows meeting the query's WHERE clause. When the planner can prove this, it excludes the partition from the query plan.

大胆强调我的。计划者不理解复杂的表达方式。当然，这也必须满足:

Ensure that the constraint_exclusion configuration parameter is not disabled in postgresql.conf. If it is, queries will not be optimized as desired.

代替

<strike>SELECT * FROM foo WHERE (id = 2);</strike>

尝试:

SELECT * FROM foo WHERE <b>id % 30 = 2 AND </b>id = 2;

和:

The default (and recommended) setting of constraint_exclusion is actually neither on nor off, but an intermediate setting called partition, which causes the technique to be applied only to queries that are likely to be working on partitioned tables. The on setting causes the planner to examine CHECK constraints in all queries, even simple ones that are unlikely to benefit.

您可以尝试使用 constraint_exclusion = on 来查看规划器是否在没有冗余逐字条件的情况下流行起来。但是您必须权衡此设置的成本和 yield 。

另一种方法是为您的分区提供更简单的条件，因为 outlined by @harmic .

不，增加 STATISTICS 的数量在这种情况下无济于事。只有 CHECK 约束和查询中的 WHERE 条件很重要。