Bruce Momjian <pgman@candle.pha.pa.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