Tom Lane wrote: > Bruce Momjian <[EMAIL PROTECTED]> 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 [EMAIL PROTECTED] | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])
