Re: Explain

On Mon, 7 Jul 2003 JEANARTHUR@EUROVOX.FR wrote:

> Hi,
>
> when I do an explain on a certain query, I have this answer :
>
>
>                                       QUERY PLAN
> --------------------------------------------------------------------------------------
>  Aggregate  (cost=100017927.48..100017927.48 rows=1 width=8)
>    ->  Seq Scan on stats_daily_2003
> (cost=100000000.00..100017927.47 rows=1 width=8)
>          Filter: ((id_compte = 29075) AND (periode = '07-07-2003'::date))
> (3 rows)
>
>
> Well, I don"t really undestand the meaning of
>
> cost=100000000.00..100017927.47
>
> and
>
> cost=100017927.48..100017927.48
>
> I guess this result is relatively bad.

This probably actually implies that you've done something like set
enable_seqscan=off at some point which makes the costs of seqscans very high
to discourage them. In any case, this plan is still expected to be much
more expensive than the below even when not taking that into account.  You
may also want to do explain analyze to get real world numbers rather than
just the estimates.

> The same query on a similar table but with an index give :
>                                                  QUERY PLAN
> --------------------------------------------------------------------------------------------
> ----------------
>  Aggregate  (cost=9.60..9.60 rows=1 width=8)
>    ->  Index Scan using ap_stats_daily_2003_save on
> stats_daily_2003_save  (cost=0.00..9.59 rows=2 width=8)
>          Index Cond: ((periode = '07-07-2003'::date) AND (id_compte =
> 29075))
> (3 rows)
>
> Could you confirm me the second explain is "better" than the first one
> ?