Re: Interesting new bug?

Tom Lane wrote:
>
> Tim Perdue <tim(at)sourceforge(dot)net> writes:
> > I'm attempting to select out of a large table (10GB) with about 4
> > million rows, and it winds up just sitting and doing "nothing" forever.
>
> > db_geocrawler=# explain SELECT * FROM tbl_mail_archive WHERE
> > fld_mail_list=0 ORDER BY fld_mailid ASC LIMIT 10 OFFSET 0;
> > NOTICE: QUERY PLAN:
>
> > Index Scan using tbl_mail_archive_pkey on tbl_mail_archive
> > (cost=0.00..6402391.68 rows=19357 width=80)
>
> Interesting. Since there's no explicit sort in the plan, I infer that
> index tbl_mail_archive_pkey is on fld_mailid, meaning that the indexscan
> yields data already sorted by fld_mailid --- otherwise a sort step would
> be needed. Evidently the optimizer is guessing that "scan in fld_mailid
> order until you have 10 rows where fld_mail_list=0" is faster than
> "find all rows with fld_mail_list=0 and then sort by fld_mailid".
>
> Since you're complaining, I guess that this is not so :-( ... but I'm
> not sure how the optimizer might be taught to guess that. What exactly
> are the indexes *on* here; how many rows are in the table; and how many
> rows satisfy fld_mail_list=0?

There is an index on fld_mail_list and there were 1093 rows that matched
out of about 4.1 million.

I wonder if this is the same problem we had before where I need to order
by fld_mail_list, fld_mailid instead of just on fld_mailid. If so, you
need to get that fixed in the optimizer.

db_geocrawler=# explain
db_geocrawler-# SELECT * FROM tbl_mail_archive WHERE
db_geocrawler-# fld_mail_list=0 ORDER BY fld_mail_list ASC,fld_mailid
ASC LIMIT 10 OFFSET 0;
NOTICE: QUERY PLAN:

Sort (cost=78282.54..78282.54 rows=19357 width=80)
-> Index Scan using idx_archive_list on tbl_mail_archive
(cost=0.00..76904.24 rows=19357 width=80)

EXPLAIN

Notice how it is now using the right index, because I am doing a sort on
fld_mail_list first.

Tim

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