Question

这是对Postgres 9.1.6的慢查询，即使最大计数为2，两行已经由主键标识:( 4.5秒）

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秒以上）

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)

如何阻止Postgres内联子查询？

背景：我有一个使用hstore和GiST index的Postgres 9.1表格。

Answer 1

我认为OFFSET 0是更好的方法，因为它更明显地表示某些奇怪的东西正在发生，并且我们不太可能改变围绕OFFSET 0的优化行为......希望如此CTE将在某些时候变得可以内联。以下解释是为了完整性;使用Seamus的答案。

对于不相关的子查询，您可以利用PostgreSQL拒绝内联WITH查询字词来将您的查询重新命名为：

WITH t AS (
    SELECT * FROM tbl WHERE id IN ('6d48fc431d21', 'd9e659e756ad')
)
SELECT COUNT(*) 
FROM t 
WHERE data ? 'building_floorspace' 
AND data ?| ARRAY['elec_mean_monthly_use', 'gas_mean_monthly_use'];

这与OFFSET 0 hack有很多相同的效果，就像offset 0 hack利用Pg优化器中的怪癖一样，人们用它来解决Pg缺乏查询提示......将它们用作查询提示。

Answer 2

显然有一个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)

诀窍是子查询中的OFFSET 0。

如何防止Postgres内联子查询？

2 个答案: