Re: count(*) slow on large tables

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.

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