Re: why does count take so long?

Tom Lane <tgl@sss.pgh.pa.us> writes:

> Yeah, but I think he's complaining about the 10sec delta for the
> aggregate on top of the 71sec to read the 8 million rows.  That
> seems high to me too.  On a 10-mil-row test table, I get
...
> in other words 26sec to do the aggregate on top of 163sec to read the
> rows.
>
> Unless Joseph's machine has a way better IO-to-CPU ratio than my little
> development machine, there's something odd about his numbers.

Why is 10s (a 14% delta) for 8M records suspicious but 26s (16% delta) for 10M
not suspicious? These results seem fairly consistent actually.

I think what the original question was is "what work does this 10s represent".
I'm curious too. Is it really just 10 million times the cpu cycles necessary
to dispatch a call to the count() aggregate state function?



PS:

> regression=# explain analyze select count(*) from foo;
>                                                     QUERY PLAN
> ------------------------------------------------------------------------------------------------------------------
>  Aggregate  (cost=22.50..22.50 rows=1 width=0) (actual time=189865.81..189865.81 rows=1 loops=1)
>    ->  Seq Scan on foo  (cost=0.00..20.00 rows=1000 width=0) (actual time=18.88..163833.61 rows=10240000 loops=1)
>  Total runtime: 189865.91 msec
> (3 rows)

Hey, you haven't analyzed your table! :)

--
greg