分区表查询仍然扫描所有分区

Question

我有一张超过十亿条记录的桌子.为了提高性能,我将其分区为30个分区.最常见的查询(id = ...)在where子句中有,所以我决定在id列上对表进行分区.

基本上,分区是以这种方式创建的:

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);
.
.
.

我跑ANALYZE了整个数据库,特别是,我id通过运行它来收集这个表的列的额外统计信息:

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,计划程序会显示它仍在扫描所有分区.constraint_exclusion设置为partition,所以这不是问题.

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)
.
.
.

我能做些什么让刨床有更好的计划？我是否还需要ALTER TABLE foo ALTER COLUMN id SET STATISTICS 10000;为所有分区运行？

编辑

在使用Erwin建议的查询更改后,计划程序仅扫描正确的分区,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

与:

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

对于非平凡的表达式,您必须在查询中重复或多或少的逐字条件,以使Postgres查询规划器理解它可以依赖于CHECK约束.即使看起来多余!

每个文件:

启用约束排除后,计划程序将检查每个分区的约束,并尝试证明不需要扫描分区,因为它不能包含符合查询WHERE子句的任何行.当规划人员可以证明这一点时,它会从查询计划中排除分区.

大胆强调我的.规划者不理解复杂的表达方式.当然,这也必须得到满足:

确保未禁用constraint_exclusion配置参数postgresql.conf.如果是,则不会根据需要优化查询.

代替

SELECT * FROM foo WHERE (id = 2);

尝试:

SELECT * FROM foo WHERE id % 30 = 2 AND id = 2;

和:

constraint_exclusion的默认(和推荐)设置实际上既不是on也不是off,而是调用的中间设置 partition,这使得该技术仅应用于可能在分区表上工作的查询.on设置使计划程序检查CHECK所有查询中的约束,即使是不太可能受益的简单查询.

您可以尝试constraint_exclusion = on查看计划程序是否在没有冗余逐字条件的情况下捕获.但是你必须权衡这个设置的成本和收益.

替代方案是@harmic已经概述的分区更简单的条件.

否,STATISTICS在这种情况下增加数量将无济于事.只有查询中的CHECK约束和WHERE条件才有意义.

Answer 2

不幸的是,在postgresql中分配是相当原始的.它仅适用于基于范围和列表的约束.您的分区约束太复杂,查询计划程序无法决定排除某些分区.

在手册中它说:

保持分区约束简单,否则规划人员可能无法证明不需要访问分区.使用简单的相等条件进行列表分区,或使用简单范围测试进行范围分区,如前面的示例所示.一个好的经验法则是,分区约束应该只包含使用B-tree-indexable运算符将分区列与常量进行比较.

正如Erwin建议的那样,你可能会改变你的WHERE子句,以便明确提到模数表达式.我过去没有太多运气,虽然我最近没有尝试过,正如他所说,计划者已经有了改进.这可能是第一件要尝试的事情.

否则,您必须重新排列分区以使用id值范围而不是现在使用的模数方法.我知道,这不是一个好的解决方案.

另一种解决方案是将id的模数存储在单独的列中,然后可以对分区约束使用简单的值相等性检查.但是,有点浪费磁盘空间,而且还需要在where子句中添加一个术语来引导.