From: | "Robert Haas" <robertmhaas(at)gmail(dot)com> |
---|---|
To: | "Greg Stark" <stark(at)enterprisedb(dot)com> |
Cc: | "Greg Smith" <gsmith(at)gregsmith(dot)com>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "ITAGAKI Takahiro" <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, Postgres <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: posix_fadvise v22 |
Date: | 2009-01-03 01:42:51 |
Message-ID: | 603c8f070901021742s61dcbaf2r51deff1219196944@mail.gmail.com |
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Lists: | pgsql-hackers |
> Hm, what were those plans? You might want to put the old code back in
> explain.c to print the prefetching target to see how well it's doing.
Well, bad news. Here's one where prefetching seems to make it WORSE.
rhaas=# explain select sum(1) from enormous where l_shipdate in
('1992-01-01', '1993-01-01', '1994-01-01', '1995-01-01', '1996-01-01',
'1997-01-01', '1998-01-01', '1999-01-01', '2000-01-01', '2001-01-01');
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------
------------
Aggregate (cost=455072.75..455072.76 rows=1 width=0)
-> Bitmap Heap Scan on enormous (cost=3327.59..454634.09
rows=175464 width=0)
Recheck Cond: (l_shipdate = ANY
('{1992-01-01,1993-01-01,1994-01-01,1995-01-01,1996-01-01,1997-01-01,1998-01-01,1999-01-01,2000-01-01,2001-01-01}'::d
ate[]))
-> Bitmap Index Scan on enormous_l_shipdate
(cost=0.00..3283.72 rows=175464 width=0)
Index Cond: (l_shipdate = ANY
('{1992-01-01,1993-01-01,1994-01-01,1995-01-01,1996-01-01,1997-01-01,1998-01-01,1999-01-01,2000-01-01,2001-01-01}
'::date[]))
(5 rows)
With effective_io_concurrency set to 1, this took 32 s. With
effective_io_concurrency set to 4, it took 50 s. The table was
created like this:
create table enormous as select l.*, l_instance from lineitem l,
generate_series(1, 8) l_instance;
create index enormous_l_shipdate on enormous (l_shipdate);
vacuum analyze enormous;
...where lineitem is from the skewed TPC-H data for the histojoin patch.
...Robert
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