Re: I have a question about using index in order statement.
От | Heikki Linnakangas |
---|---|
Тема | Re: I have a question about using index in order statement. |
Дата | |
Msg-id | 472B1000.6000208@enterprisedb.com обсуждение исходный текст |
Ответ на | I have a question about using index in order statement. ("kevin" <kevin@mail.kinew.com>) |
Ответы |
Re: I have a question about using index in order statement.
(Tom Lane <tgl@sss.pgh.pa.us>)
|
Список | pgsql-bugs |
kevin wrote: > Question: > I have a question about using index in order statement. > Why index ix_2 work by Seq Scan and index ix_3 work by Index Scan. > > Example : > > ix_2 condition : > When I try > > explain > select * from a_test > order by code_ desc > > Postgresql response > Sort (cost=100001815.08..100001852.56 rows=14990 width=56) > Sort Key: code_ > -> Seq Scan on a_test (cost=100000000.00..100000260.90 rows=14990 width=56) > > ix_3 condition : > When I try > > explain > select * from a_test > order by lower(code_) desc > > Postgresql response > Index Scan using ix_3 on a_test (cost=0.00..769.27 rows=14990 width=18) Thanks for the report. This seems to have been broken by this patch back in May: http://archives.postgresql.org/pgsql-committers/2007-05/msg00394.php that wraps pathkey expressions with a relabel node. Because of that, get_eclass_for_sort_expr doesn't recognize that the ordering of the index matches that of the query. Attached is a patch that fixes that test case. I'm not very familiar with that piece of code, though, and I have a sneaking suspicion that the patch is either not general enough, there may be other places where we should ignore relabel nodes, or it brakes something else. I'm surprised this hasn't been noticed before. It doesn't happen with text datatype, but varchar is very common datatype as well. PS. Kevin, in the future, please specify which PostgreSQL version you're using. The fact that the above DDL statements don't work until 8.3beta releases gave it away this time :-). -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com Index: src/backend/optimizer/path/equivclass.c =================================================================== RCS file: /home/hlinnaka/pgcvsrepository/pgsql/src/backend/optimizer/path/equivclass.c,v retrieving revision 1.3 diff -c -r1.3 equivclass.c *** src/backend/optimizer/path/equivclass.c 7 Jul 2007 20:46:45 -0000 1.3 --- src/backend/optimizer/path/equivclass.c 2 Nov 2007 11:48:12 -0000 *************** *** 373,378 **** --- 373,388 ---- EquivalenceMember *newem; ListCell *lc1; MemoryContext oldcontext; + Expr *stripped_expr; + + /* + * Strip any relabel nodes first; they're not meaningful + * for ordering purposes. + */ + if (IsA(expr, RelabelType)) + stripped_expr = ((RelabelType *)expr)->arg; + else + stripped_expr = expr; /* * Scan through the existing EquivalenceClasses for a match *************** *** 390,395 **** --- 400,406 ---- foreach(lc2, cur_ec->ec_members) { EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2); + Expr *em_expr; /* * If below an outer join, don't match constants: they're not *************** *** 399,406 **** cur_em->em_is_const) continue; if (expr_datatype == cur_em->em_datatype && ! equal(expr, cur_em->em_expr)) return cur_ec; /* Match! */ } } --- 410,421 ---- cur_em->em_is_const) continue; + em_expr = cur_em->em_expr; + if (IsA(em_expr, RelabelType)) + em_expr = ((RelabelType *)em_expr)->arg; + if (expr_datatype == cur_em->em_datatype && ! equal(stripped_expr, em_expr)) return cur_ec; /* Match! */ } }
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