Re: count(*) slow on large tables

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Tom Lane wrote:
> >> ... 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.
>
> Oh, that idea.  Yeah, I think we had concluded it might work.  You'd
> better make the TODO item link to that discussion, because there's sure
> been plenty of discussion of ideas that wouldn't work.

OK, I beefed up the TODO:

    * Use a fixed row count and a +/- count with MVCC visibility rules
      to allow fast COUNT(*) queries with no WHERE clause(?)

I can always give the details if someone asks.  It doesn't seem complex
enough for a separate TODO.detail item.

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