如何防止 Postgres 内联子查询? [英] How can I prevent Postgres from inlining a subquery?

问题描述

这是一个关于 Postgres 9.1.6 的慢查询，即使最大计数为 2，两行都已由它们的主键标识:(4.5 秒)

Here's a slow query on Postgres 9.1.6, even though the maximum count is 2, with both rows already identified by their primary keys: (4.5 seconds)

EXPLAIN ANALYZE SELECT COUNT(*) FROM tbl WHERE id IN ('6d48fc431d21', 'd9e659e756ad') AND data ? 'building_floorspace' AND data ?| ARRAY['elec_mean_monthly_use', 'gas_mean_monthly_use'];
                                                                     QUERY PLAN                                                                     
----------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=4.09..4.09 rows=1 width=0) (actual time=4457.886..4457.887 rows=1 loops=1)
   ->  Index Scan using idx_tbl_on_data_gist on tbl  (cost=0.00..4.09 rows=1 width=0) (actual time=4457.880..4457.880 rows=0 loops=1)
         Index Cond: ((data ? 'building_floorspace'::text) AND (data ?| '{elec_mean_monthly_use,gas_mean_monthly_use}'::text[]))
         Filter: ((id)::text = ANY ('{6d48fc431d21,d9e659e756ad}'::text[]))
 Total runtime: 4457.948 ms
(5 rows)

嗯，也许如果我先用主键部分做一个子查询......:(不，仍然是 4.5+ 秒)

Hmm, maybe if I do a subquery with just the primary key part first...: (nope, still 4.5+ seconds)

EXPLAIN ANALYZE SELECT COUNT(*) FROM (  SELECT * FROM tbl WHERE id IN ('6d48fc431d21', 'd9e659e756ad')  ) AS t WHERE data ? 'building_floorspace' AND data ?| ARRAY['elec_mean_monthly_use', 'gas_mean_monthly_use'];
                                                                     QUERY PLAN                                                                     
----------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=4.09..4.09 rows=1 width=0) (actual time=4854.170..4854.171 rows=1 loops=1)
   ->  Index Scan using idx_tbl_on_data_gist on tbl  (cost=0.00..4.09 rows=1 width=0) (actual time=4854.165..4854.165 rows=0 loops=1)
         Index Cond: ((data ? 'building_floorspace'::text) AND (data ?| '{elec_mean_monthly_use,gas_mean_monthly_use}'::text[]))
         Filter: ((id)::text = ANY ('{6d48fc431d21,d9e659e756ad}'::text[]))
 Total runtime: 4854.220 ms
(5 rows)

如何防止 Postgres 内联子查询?

How can I prevent Postgres from inlining the subquery?

背景:我有一个 Postgres 9.1 表，使用 hstore 和 GiST 索引就可以了.

Background: I have a Postgres 9.1 table using hstore and with a GiST index on it.

推荐答案

显然有一个 告诉 Postgres 不要内联的方法:(0.223ms！)

Apparently there's a way to tell Postgres not to inline: (0.223ms!)

EXPLAIN ANALYZE SELECT COUNT(*) FROM (  SELECT * FROM tbl WHERE id IN ('6d48fc431d21', 'd9e659e756ad') OFFSET 0 ) AS t WHERE data ? 'building_floorspace' AND data ?| ARRAY['elec_mean_monthly_use', 'gas_mean_monthly_use'];
                                                                QUERY PLAN                                                                
------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=8.14..8.15 rows=1 width=0) (actual time=0.165..0.166 rows=1 loops=1)
   ->  Subquery Scan on t  (cost=4.14..8.14 rows=1 width=0) (actual time=0.160..0.160 rows=0 loops=1)
         Filter: ((t.data ? 'building_floorspace'::text) AND (t.data ?| '{elec_mean_monthly_use,gas_mean_monthly_use}'::text[]))
         ->  Limit  (cost=4.14..8.13 rows=2 width=496) (actual time=0.086..0.092 rows=2 loops=1)
               ->  Bitmap Heap Scan on tbl  (cost=4.14..8.13 rows=2 width=496) (actual time=0.083..0.086 rows=2 loops=1)
                     Recheck Cond: ((id)::text = ANY ('{6d48fc431d21,d9e659e756ad}'::text[]))
                     ->  Bitmap Index Scan on tbl_pkey  (cost=0.00..4.14 rows=2 width=0) (actual time=0.068..0.068 rows=2 loops=1)
                           Index Cond: ((id)::text = ANY ('{6d48fc431d21,d9e659e756ad}'::text[]))
 Total runtime: 0.223 ms
(9 rows)

技巧是子查询中的OFFSET 0.

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