Interesting new bug?

Allright, I'm running 7.0.2 with Tom Lane's backwards index scan patch
applied.

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.
If I check the process list, I see it using about 9% of the CPU.

This table is vacuum analyzed nightly - here's a description and EXPLAIN
from the query I'm trying to run.

Any ideas? I haven't been able to run the admin pages on Geocrawler ever
since I upgraded to 7.0.2

Tim

db_geocrawler=# \d tbl_mail_archive
Table "tbl_mail_archive"
Attribute | Type |
Modifier
----------------------+----------+----------------------------------------------
fld_mailid | integer | not null default
nextval('seq_mailid'::text)
fld_mail_list | integer |
fld_mail_date | char(14) |
fld_mail_is_followup | integer |
fld_mail_from | text |
fld_mail_subject | text |
fld_mail_body | text |
fld_mail_email | text |
fld_mail_year | integer |
fld_mail_month | integer |
Indices: idx_archive_list,
idx_archive_list_date,
idx_archive_year,
idx_mail_archive_list_yr_mo,
tbl_mail_archive_pkey

I'm manually deleting the rows without knowing what they are - and
that's bad - this query shows that the rows do exist, but for some
reason you can't select them out of the db.

db_geocrawler=# begin;
BEGIN
db_geocrawler=# delete from tbl_mail_archive where fld_mail_list=0;
DELETE 1032
db_geocrawler=# delete from tbl_mail_chunks where fld_mail_list=0;
DELETE 39
db_geocrawler=# commit;
COMMIT

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)

EXPLAIN

--
Founder - PHPBuilder.com / Geocrawler.com
Lead Developer - SourceForge
VA Linux Systems
408-542-5723

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?

regards, tom lane

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

--
Founder - PHPBuilder.com / Geocrawler.com
Lead Developer - SourceForge
VA Linux Systems
408-542-5723

What did you think of this? I fixed my problem by changing my query -
but I shouldn't have had to. This looks like a weakness in your
optimizer, having to first sort on criteria that you don't care about.

Tim

Tim Perdue wrote:
>
> 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.

--
Founder - PHPBuilder.com / Geocrawler.com
Lead Developer - SourceForge
VA Linux Systems
408-542-5723