Re: Query Plan choice with timestamps

On 07/ago/08, at 17:50, Tom Lane wrote:

> Giorgio Valoti <giorgio_v(at)mac(dot)com> writes:
>> GroupAggregate (cost=98431.58..119773.92 rows=74226 width=8)
>> -> Sort (cost=98431.58..99050.92 rows=247736 width=8)
>> Sort Key: (day_trunc(ts))
>> -> Seq Scan on blackbox (cost=0.00..72848.36 rows=247736
>> width=8)
>
>> GroupAggregate (cost=0.00..19109.66 rows=74226 width=8)
>> -> Index Scan using test_2_idx on blackbox
>> (cost=0.00..16943.16 rows=247736 width=8)
>
> These numbers seem pretty bogus: there is hardly any scenario in
> which a
> full-table indexscan should be costed as significantly cheaper than a
> seqscan. Have you put in silly values for random_page_cost?

No,

>
>
> If you haven't mucked with the cost parameters, the only way I can
> think
> of to get this result is to have an enormously bloated table that's
> mostly empty. Maybe you need to review your vacuuming procedures.

I’ll review them.

Thank you
--
Giorgio Valoti