From: Achilleas Mantzios <achill@matrix.gatewaynet.com>
Organization: Dynacom Tankers Mgmt
To: pgsql-admin@postgresql.org
Subject: Postgresql 9.0.2 explain analyze very slow (10x),
	compared to actual query time
Date: Tue, 11 Jan 2011 17:53:22 +0200
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Hello,
I have noticed that in 9.0.2 EXPLAIN ANALYZE takes considerably longer time to return than the actual query. I never noticed this in 8.3.
Example:
In 9.0.2 i get
dynacom=# EXPLAIN ANALYZE SELECT count(ms.id) from marinerstates ms,mariner m where ms.endtime IS NOT NULL AND ms.marinerid=m.id;
                                                            QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=4716.48..4716.49 rows=1 width=4) (actual time=413.378..413.379 rows=1 loops=1)
   ->  Hash Join  (cost=788.77..4582.04 rows=53775 width=4) (actual time=54.029..330.286 rows=53845 loops=1)
         Hash Cond: (ms.marinerid = m.id)
         ->  Seq Scan on marinerstates ms  (cost=0.00..2583.33 rows=53775 width=8) (actual time=0.005..99.092 rows=53845 loops=1)
               Filter: (endtime IS NOT NULL)
         ->  Hash  (cost=583.90..583.90 rows=16390 width=4) (actual time=54.011..54.011 rows=16390 loops=1)
               Buckets: 2048  Batches: 1  Memory Usage: 577kB
               ->  Seq Scan on mariner m  (cost=0.00..583.90 rows=16390 width=4) (actual time=0.004..27.587 rows=16390 loops=1)
 Total runtime: 413.495 ms

dynacom=# \timing
Timing is on.
dynacom=# SELECT count(ms.id) from marinerstates ms,mariner m where ms.endtime IS NOT NULL AND ms.marinerid=m.id;
 count
-------
 53845
(1 row)

Time: 39,038 ms

So the actual time (39 ms) differs considerably from the EXPLAIN ANALYZE Total runtime (413 ms) (10 times bigger)

In 8.3 i get
postgres@dynacom=# EXPLAIN ANALYZE SELECT count(ms.id) from marinerstates ms,mariner m where ms.endtime IS NOT NULL AND ms.marinerid=m.id;
                                                            QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=5094.13..5094.14 rows=1 width=4) (actual time=84.180..84.180 rows=1 loops=1)
   ->  Hash Join  (cost=869.15..4954.27 rows=55942 width=4) (actual time=11.284..72.630 rows=54168 loops=1)
         Hash Cond: (ms.marinerid = m.id)
         ->  Seq Scan on marinerstates ms  (cost=0.00..2756.50 rows=55942 width=8) (actual time=0.013..25.696 rows=54168 loops=1)
               Filter: (endtime IS NOT NULL)
         ->  Hash  (cost=642.40..642.40 rows=18140 width=4) (actual time=11.256..11.256 rows=16442 loops=1)
               ->  Seq Scan on mariner m  (cost=0.00..642.40 rows=18140 width=4) (actual time=0.007..5.762 rows=16442 loops=1)
 Total runtime: 84.346 ms
(8 rows)

postgres@dynacom=# SELECT count(ms.id) from marinerstates ms,mariner m where ms.endtime IS NOT NULL AND ms.marinerid=m.id;
 count
-------
 54168
(1 row)

Time: 42.224 ms

In 8.3 the Total runtime is only double the value of the actual time.

Is there anything that can shed some light on this?

-- 
Achilleas Mantzios