From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Christopher Browne <cbbrowne(at)libertyrms(dot)info>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: count(*) slow on large tables |
Date: | 2003-10-04 17:48:47 |
Message-ID: | 200310041748.h94Hmlc04602@candle.pha.pa.us |
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Lists: | pgsql-hackers pgsql-performance |
Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > We do have a TODO item:
> > * Consider using MVCC to cache count(*) queries with no WHERE clause
>
> > The idea is to cache a recent count of the table, then have
> > insert/delete add +/- records to the count. A COUNT(*) would get the
> > main cached record plus any visible +/- records. This would allow the
> > count to return the proper value depending on the visibility of the
> > requesting transaction, and it would require _no_ heap or index scan.
>
> ... and it would give the wrong answers. Unless the cache is somehow
> snapshot-aware, so that it can know which other transactions should be
> included in your count.
The cache is an ordinary table, with xid's on every row. I meant it
would require no index/heap scans of the large table --- it would still
require a scan of the "count" table.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
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