| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Bruce Momjian <pgman(at)candle(dot)pha(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 16:49:33 |
| Message-ID: | 6743.1065286173@sss.pgh.pa.us |
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| Lists: | pgsql-hackers pgsql-performance |
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.
regards, tom lane
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