How can I prevent Postgres from inlining a subquery?

Question

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 AN         


        

Answer 1

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.