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Re: [BUGS] BUG #8049: Incorrect results when using ORDER BY and query planner options - Mailing list pgsql-hackers

From Jov
Subject Re: [BUGS] BUG #8049: Incorrect results when using ORDER BY and query planner options
Date
Msg-id CADyrUxM3ZRMEPUHffkaZ0q9zqVCopQvWeCcbpuEaF34badzX7Q@mail.gmail.com
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In response to Re: [BUGS] BUG #8049: Incorrect results when using ORDER BY and query planner options  ("Dickson S. Guedes" <listas@guedesoft.net>)
Responses Re: [BUGS] BUG #8049: Incorrect results when using ORDER BY and query planner options
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I can  reproduce on 9.2.4 too.

it seams a filter was incorrect push down, I find a SQL produce the same plan,but filter in the inner query.

 test=# explain analyze SELECT * FROM
(
        SELECT (COALESCE(h_n::varchar , '') || l_n) AS fault
        FROM
        (
                SELECT _bug_header.h_n, _bug_line.l_n
                FROM _bug_line
                LEFT OUTER JOIN _bug_header on (_bug_line.h_n = _bug_header.h_n)
        ) AS tmp
) AS tmp2
        WHERE fault = '1'
ORDER BY
     1;
 Nested Loop Left Join  (cost=10000000000.00..10000000687.59 rows=11 width=8) (actual time=0.017..0.024 rows
=2 loops=1)
   ->  Seq Scan on _bug_line  (cost=10000000000.00..10000000031.40 rows=2140 width=8) (actual time=0.005..0.
006 rows=2 loops=1)
   ->  Index Only Scan using _bug_header_unique on _bug_header  (cost=0.00..0.30 rows=1 width=4) (actual tim
e=0.002..0.002 rows=0 loops=2)
         Index Cond: (h_n = _bug_line.h_n)
         Filter: (((COALESCE((h_n)::character varying, ''::character varying))::text || (_bug_line.l_n)::tex
t) = '1'::text)
         Heap Fetches: 0
 Total runtime: 0.059 ms

it produce same query plan and result with this SQL:

 explain analyze SELECT * FROM
(
        SELECT (COALESCE(h_n::varchar , '') || l_n) AS fault
        FROM
        (
                SELECT _bug_header.h_n, _bug_line.l_n
                FROM _bug_line
                LEFT OUTER JOIN _bug_header on (_bug_line.h_n = _bug_header.h_n and (COALESCE(_bug_header.h_n::varchar , '') || _bug_line.l_n) = '1')
        ) AS tmp
) AS tmp2;

 Nested Loop Left Join  (cost=10000000000.00..10000000714.21 rows=2140 width=8) (actual time=0.015..0.024 ro
ws=2 loops=1)
   ->  Seq Scan on _bug_line  (cost=10000000000.00..10000000031.40 rows=2140 width=8) (actual time=0.005..0.
007 rows=2 loops=1)
   ->  Index Only Scan using _bug_header_unique on _bug_header  (cost=0.00..0.30 rows=1 width=4) (actual tim
e=0.002..0.002 rows=0 loops=2)
         Index Cond: (h_n = _bug_line.h_n)
         Filter: (((COALESCE((h_n)::character varying, ''::character varying))::text || (_bug_line.l_n)::tex
t) = '1'::text)
         Heap Fetches: 0
 Total runtime: 0.058 ms

but after analyze both tables,the problem is gone.

so, less accurate stat data can not only produce less efficient query plan, but also wrong plan. I think it is a terrible bug.  


2013/4/10 Dickson S. Guedes <listas@guedesoft.net>
2013/4/9  <th@atsc.nl>:
> I've got a strange problem with a query that produces more results than
> expected.

I tested this [1] and saw that 9.1 don't has the behavior then I
started bisect from REL9_1_9 to HEAD and found that commit 5b7b5518d0e
[2] introduced it.

I'm putting a copy to -hacker list in a hope that this help some one
with more experience on that code to go further.

[1] http://www.postgresql.org/message-id/CAHHcrepFavBPnTSE9Ho3=8FgXH8ciR7byKyMGMbdDfLdpWfzng@mail.gmail.com
[2] http://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=5b7b5518d0ea56c422a197875f7efa5deddbb388

[]s
--
Dickson S. Guedes
mail/xmpp: guedes@guedesoft.net - skype: guediz
http://github.com/guedes - http://guedesoft.net
http://www.postgresql.org.br


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--
Jov

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