Incorrect behavior with CE and ORDER BY

"Joshua D. Drake" <jd@commandprompt.com> writes:

We have a problem with CE that I want to verify is either expected
behavior, a bug or something else :).

Uh, what's your problem exactly? The example only seems to demonstrate
that if you don't ask for a sort, you don't get one.

regards, tom lane

Tom Lane wrote:

"Joshua D. Drake" <jd@commandprompt.com> writes:

We have a problem with CE that I want to verify is either expected
behavior, a bug or something else :).

Uh, what's your problem exactly? The example only seems to demonstrate
that if you don't ask for a sort, you don't get one.

Sorry. The problem is, if I ask for an ORDER BY it scans all partitions
versus only scanning the partition that has the data in it.

Sincerely,

Joshua D. Drake

regards, tom lane

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Joshua D. Drake wrote:

Tom Lane wrote:

"Joshua D. Drake" <jd@commandprompt.com> writes:

We have a problem with CE that I want to verify is either expected
behavior, a bug or something else :).

Uh, what's your problem exactly? The example only seems to demonstrate
that if you don't ask for a sort, you don't get one.

Sorry. The problem is, if I ask for an ORDER BY it scans all partitions
versus only scanning the partition that has the data in it.

Huh, but that's not what the EXPLAIN ANALYZE you posted says ...

--
Alvaro Herrera http://www.CommandPrompt.com/
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Alvaro Herrera wrote:

Joshua D. Drake wrote:

Tom Lane wrote:

"Joshua D. Drake" <jd@commandprompt.com> writes:

We have a problem with CE that I want to verify is either expected
behavior, a bug or something else :).

Uh, what's your problem exactly? The example only seems to demonstrate
that if you don't ask for a sort, you don't get one.

Sorry. The problem is, if I ask for an ORDER BY it scans all partitions
versus only scanning the partition that has the data in it.

Huh, but that's not what the EXPLAIN ANALYZE you posted says ...

Sorry I realize the error of my ways. It isn't that it is scanning all
partitions, it is that it is scanning all of a single partition (subject
to the WHERE clause). That is correct behavior.

Sincerely,

Joshua D. Drake

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I followed up with Joshua on Jabber. This is the query:

SELECT pse_userid FROM tbl_profile_search WHERE pse_normalized_text='1'
and pse_interest_type = 10 order by pse_lastlogin DESC limit 50 offset 0

I suggested adding an index on (pse_normalized_text, pse_lastlogin), on
the assumption that the planner would get the sorted output from there
and be able to push the LIMIT clause, just below the indexscan, thus
saving the big heap scan (and a sort across a large result set). But it
turns out the index is already there.

So it seems to me to be a planner shortcoming. Is this correct?

My idea of the plan would be (tabs=8 spaces)

Limit (50)
Sort (key: pse_lastlogin)
Result
Append
SeqScan tbl_profile_search
Indexscan tbl_profile_search_interest_1
Limit (50)
IndexScan on the index mentioned above

Is this possible? It would be very fast. Maybe it should be like this
instead:

Limit (50)
Sort (key: pse_lastlogin)
Result
Append
Limit (50)
SeqScan tbl_profile_search
Limit (50)
Indexscan tbl_profile_search_interest_1
Limit (50)
IndexScan on the index mentioned above

This is the actual plan:

Limit (cost=47110.19..47110.31 rows=50 width=8) (actual
time=6088.013..6088.269 rows=50 loops=1)
-> Sort (cost=47110.19..47943.14 rows=333179 width=8) (actual
time=6088.007..6088.104 rows=50 loops=1)
Sort Key: public.tbl_profile_search.pse_lastlogin
-> Result (cost=0.00..16547.78 rows=333179 width=8) (actual
time=0.020..4339.472 rows=334319 loops=1)
-> Append (cost=0.00..16547.78 rows=333179 width=8)
(actual time=0.016..3208.022 rows=334319 loops=1)
-> Seq Scan on tbl_profile_search
(cost=0.00..2.27 rows=1 width=8) (actual time=0.012..0.047 rows=2 loops=1)
Filter: (((pse_normalized_text)::text =
'1'::text) AND (pse_interest_type = 10))
-> Index Scan using index_pse_09_on_part_1 on
tbl_profile_search_interest_1 tbl_profile_search (cost=0.00..4.73
rows=1 width=8) (actual time=0.202..0.202 rows=0 loops=1)
Index Cond: ((pse_normalized_text)::text =
'1'::text)
Filter: (pse_interest_type = 10)
-> Bitmap Heap Scan on
tbl_profile_search_interest_10 tbl_profile_search
(cost=3579.12..16540.78 rows=333177 width=8) (actual
time=90.619..2116.224 rows=334317 loops=1)
Recheck Cond: ((pse_normalized_text)::text =
'1'::text)
Filter: (pse_interest_type = 10)
-> Bitmap Index Scan on
index_pse_09_on_part_10 (cost=0.00..3579.12 rows=333177 width=0)
(actual time=89.052..89.052 rows=340964 loops=1)
Index Cond:
((pse_normalized_text)::text = '1'::text)
Total runtime: 6103.190 ms

--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

Alvaro Herrera <alvherre@commandprompt.com> writes:

Is this possible? It would be very fast.

It's possible but not exactly simple. As an example, your proposed
plan:

Limit (50)
Sort (key: pse_lastlogin)
Result
Append
Limit (50)
SeqScan tbl_profile_search
Limit (50)
Indexscan tbl_profile_search_interest_1
Limit (50)
IndexScan on the index mentioned above

is wrong because there's no guarantee that the first 50 elements of a
seqscan will be anything special. You could imagine dealing with that
by sorting the seqscan results and limiting to 50, or by not
sorting/limiting that data at all but letting the upper sort see all the
seqscan entries. Offhand I think either of those could win depending on
how many elements the seqscan will yield. Also, it might be interesting
to consider inventing a "merge" plan node type that takes N
already-sorted inputs and produces a sorted output stream. Then we'd
need to trade off this approach versus doing the top-level sort, which
could cope with some of its inputs not being pre-sorted.

This seems to have some aspects in common with the recent discussion
about how to optimize min/max aggregates across an appendrel set.

regards, tom lane

Tom Lane ha scritto:

Alvaro Herrera <alvherre@commandprompt.com> writes:

Limit (50)
Sort (key: pse_lastlogin)
Result
Append
Limit (50)
SeqScan tbl_profile_search
Limit (50)
Indexscan tbl_profile_search_interest_1
Limit (50)
IndexScan on the index mentioned above

is wrong because there's no guarantee that the first 50 elements of a
seqscan will be anything special. You could imagine dealing with that
by sorting the seqscan results and limiting to 50, or by not
sorting/limiting that data at all but letting the upper sort see all the
seqscan entries. Offhand I think either of those could win depending on
how many elements the seqscan will yield. Also, it might be interesting
to consider inventing a "merge" plan node type that takes N
already-sorted inputs and produces a sorted output stream. Then we'd
need to trade off this approach versus doing the top-level sort, which
could cope with some of its inputs not being pre-sorted.

This seems to have some aspects in common with the recent discussion
about how to optimize min/max aggregates across an appendrel set.

The plan proposed by Alvaro reminds me of:

http://archives.postgresql.org/pgsql-performance/2005-09/msg00047.php

My proposal was in fact (Alvaro's plan + first Tom's suggested change):

Limit (50)
Sort (key: pse_lastlogin)
Result
Append
Limit (50)
Sort (key: pse_lastlogin)
SeqScan tbl_profile_search
Limit (50)
Indexscan tbl_profile_search_interest_1
Limit (50)
IndexScan on the index mentioned above

The plan was generated rewriting the query to use explicit subselect and
forcing the planner to order by and limit for each subquery.

I've tried a few times to write a patch to handle it, but I wasn't able
to do it because of my lack of internals knowledge and spare time.

Best regards
--
Matteo Beccati
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