How can I prevent Postgres from inlining a subquery?

问题

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)

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)

How can I prevent Postgres from inlining the subquery?

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

回答1:

I think OFFSET 0 is the better approach since it's more obviously a hack showing that something weird is going on, and it's unlikely we'll ever change the optimiser behaviour around OFFSET 0 ... wheras hopefully CTEs will become inlineable at some point. The following explanation is for completeness's sake; use Seamus's answer.

For uncorrelated subqueries you can exploit PostgreSQL's refusal to inline WITH query terms to rephrase your query as:

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'];

This has much the same effect as the OFFSET 0 hack, and like the offset 0 hack exploits quirks in Pg's optimizer that people use to get around Pg's lack of query hints ... by using them as query hints.

回答2:

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)

The trick is OFFSET 0 in the subquery.

来源：https://stackoverflow.com/questions/14897816/how-can-i-prevent-postgres-from-inlining-a-subquery

标签

postgresql

indexing

subquery

inlining