Christopher Browne wrote:
> jllachan@nsd.ca (Jean-Luc Lachance) writes:
> > That's one of the draw back of MVCC.
> > I once suggested that the transaction number and other house keeping
> > info be included in the index, but was told to forget it...
> > It would solve once and for all the issue of seq_scan vs index_scan.
> > It would simplify the aggregate problem.
>
> It would only simplify _one_ case, namely the case where someone cares
> about the cardinality of a relation, and it would do that at
> _considerable_ cost.
>
> A while back I outlined how this would have to be done, and for it to
> be done efficiently, it would be anything BUT simple.
>
> It would be very hairy to implement it correctly, and all this would
> cover is the single case of "SELECT COUNT(*) FROM SOME_TABLE;"
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.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
