Oops! pgman@candle.pha.pa.us (Bruce Momjian) was seen spray-painting on a wall:
> Neil Conway wrote:
>> On Thu, 2003-09-04 at 22:02, Bruce Momjian wrote:
>> > My idea is that if a transaction doing a COUNT(*) would first look to
>> > see if there already was a visible cached value, and if not, it would do
>> > the COUNT(*) and insert into the cache table. Any INSERT/DELETE would
>> > remove the value from the cache. As I see it, the commit of the
>> > INSERT/DELETE transaction would then auto-invalidate the cache at the
>> > exact time the transaction commits. This would allow MVCC visibility of
>> > the counts.
>>
>> But this means that some of the time (indeed, *much* of the time),
>> COUNT(*) would require a seqscan of the entire table. Since at many
>> sites that will take an enormous amount of time (and disk I/O),
>> that makes this solution infeasible IMHO.
>>
>> In general, I don't think this is worth doing.
>
> It is possible it isn't worth doing. Can the INSERT/DELETE
> incrementing/decrementing the cached count work reliabily?
Wouldn't this more or less be the same thing as having a trigger that
does, upon each insert/delete "update pg_counts set count = count + 1
where reltable = 45232;"? (... where 1 would be -1 for deletes, and where
45232 is the OID of the table...)
Technically, it seems _feasible_, albeit with the problem that it
turns pg_counts into a pretty horrid bottleneck. If lots of backends
are updating that table, then row 45232 in pg_counts becomes
troublesome because all those processes have to serialize around
updating it.
And if I have tables where I insert lots of data, but couldn't care
less how many rows they have, this effort is wasted.
When I was curious as to how COUNT might be maintained, I was pretty
sure that this wouldn't be the preferred method...
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