-----Original Message----- From: pgsql-performance-ow...@postgresql.org [mailto:pgsql-performance-ow...@postgresql.org] On Behalf Of David G Johnston Sent: Friday, September 12, 2014 12:45 PM To: pgsql-performance@postgresql.org Subject: Re: [PERFORM] weird execution plan
Huang, Suya wrote > Can someone figure out why the first query runs so slow comparing to > the second one? They generate the same result... Try: EXPLAIN (ANALYZE, BUFFERS) I believe you are only seeing caching effects. David J. -- View this message in context: http://postgresql.1045698.n5.nabble.com/weird-execution-plan-tp5818730p5818733.html Sent from the PostgreSQL - performance mailing list archive at Nabble.com. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance ================================================================================================================= Both queries have been run several times so cache would have same effect on both of them? Below is the plan with buffer information. dev=# explain (ANALYZE, BUFFERS) with x as (select distinct wid from terms_weekly_20140503 a join port_terms b on a.term=b.terms) select count(*) from x; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------------------------------ Aggregate (cost=2226187.62..2226187.63 rows=1 width=0) (actual time=3533.998..3533.999 rows=1 loops=1) Buffers: shared hit=86752 read=68837 CTE x -> HashAggregate (cost=2226181.12..2226183.12 rows=200 width=516) (actual time=3383.700..3448.554 rows=212249 loops=1) Buffers: shared hit=86752 read=68837 -> Hash Join (cost=37.67..2095240.22 rows=52376358 width=516) (actual time=0.799..3010.906 rows=1067696 loops=1) Hash Cond: (a.term = b.terms) Buffers: shared hit=86752 read=68837 -> Seq Scan on terms_weekly_20140503 a (cost=0.00..240738.81 rows=8516481 width=548) (actual time=0.023..1277.352 rows=8516481 loops=1) Buffers: shared hit=86739 read=68835 -> Hash (cost=22.30..22.30 rows=1230 width=32) (actual time=0.699..0.699 rows=1000 loops=1) Buckets: 1024 Batches: 1 Memory Usage: 51kB Buffers: shared hit=7 -> Seq Scan on port_terms b (cost=0.00..22.30 rows=1230 width=32) (actual time=0.009..0.270 rows=1000 loops=1) Buffers: shared hit=7 -> CTE Scan on x (cost=0.00..4.00 rows=200 width=0) (actual time=3383.707..3518.884 rows=212249 loops=1) Buffers: shared hit=86752 read=68837 Total runtime: 3541.277 ms (18 rows) Time: 3552.505 ms dev=# explain (analyze,buffers) select count(distinct w_id) from terms_weekly_20140503 a join port_terms b on a.term=b.terms; QUERY PLAN --------------------------------------------------------------------------------------------------------------------------------------------------------- Aggregate (cost=2226181.12..2226181.13 rows=1 width=516) (actual time=18914.881..18914.882 rows=1 loops=1) Buffers: shared hit=155589 -> Hash Join (cost=37.67..2095240.22 rows=52376358 width=516) (actual time=0.802..2616.410 rows=1067696 loops=1) Hash Cond: (a.term = b.terms) Buffers: shared hit=155589 -> Seq Scan on terms_weekly_20140503 a (cost=0.00..240738.81 rows=8516481 width=548) (actual time=0.010..966.380 rows=8516481 loops=1) Buffers: shared hit=155574 -> Hash (cost=22.30..22.30 rows=1230 width=32) (actual time=0.729..0.729 rows=1000 loops=1) Buckets: 1024 Batches: 1 Memory Usage: 51kB Buffers: shared hit=7 -> Seq Scan on port_terms b (cost=0.00..22.30 rows=1230 width=32) (actual time=0.009..0.300 rows=1000 loops=1) Buffers: shared hit=7 Total runtime: 18914.933 ms (13 rows) Time: 18915.712 ms -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
