Обсуждение: multiple index search with postgres7.1.3 on solaris 8
Hi everybody, I'm sorry if this topic has already been explained, but the search engine at archives.postgresql.org shows me 10 pages of results but without any link (!). I'm using postgres 7.1.3 on Solaris 8; I would like to do some query of this form SELECT * FROM table WHERE (attribute1,attribute2) IN ((value1_0,value2_0),(value1_1,value2_1) ...) (which is legal on oracle 8i on solaris 8). I know this has an equivalent boolean expression but: -I would like to use an index defined likeCREATE INDEX myindex ON table(attribute1,attribute2) -the pairs in the list can be many (up to 100) Postgres supports queries of the form SELECT * FROM table WHERE (attribute1,attribute2) IN (subselect) and SELECT * FROM table WHERE (attribute1,attribute2) = (value1,value2) (which strangely is not supported on oracle!). So I could use the former with a temp table *but* while the latter, according to EXPLAIN, uses the index the former doesn't. Why? Thanks in advance for any help,Marco -- Marco Vezzoli tel. +39 039 603 6852 STMicroelectronics fax. +39 039 603 5055
On Thu, Jun 12, 2003 at 13:21:01 +0200, Marco Vezzoli <marco.vezzoli@st.com> wrote: > Hi everybody, > I'm sorry if this topic has already been explained, but the search > engine at archives.postgresql.org shows me 10 pages of results but > without any link (!). > I'm using postgres 7.1.3 on Solaris 8; I would like to do some query of > this form > > SELECT * FROM table WHERE (attribute1,attribute2) IN > ((value1_0,value2_0),(value1_1,value2_1) ...) > > (which is legal on oracle 8i on solaris 8). > I know this has an equivalent boolean expression but: > -I would like to use an index defined like > CREATE INDEX myindex ON table(attribute1,attribute2) > -the pairs in the list can be many (up to 100) or'ing IN terms together is probably going to result in the same plan that you are describing above (assuming table has a large enough number of rows). I don't know if this way of writing the query can generate a sort of values you are checking against and then a merge join. For just a hundred or so values I don't think this plan would be that much better than both nest loop and multiple index scans. However, if you want to get that plan and or'ing INs won't generate it, then you could try union'ing the values together in the IN value list. 7.4 will likely behave differently than pre 7.4 versions.
bruno@wolff.to wrote: > > On Thu, Jun 12, 2003 at 13:21:01 +0200, > Marco Vezzoli <marco.vezzoli@st.com> wrote: > > Hi everybody, > > I'm sorry if this topic has already been explained, but the search > > engine at archives.postgresql.org shows me 10 pages of results but > > without any link (!). > > I'm using postgres 7.1.3 on Solaris 8; I would like to do some query of > > this form > > > > SELECT * FROM table WHERE (attribute1,attribute2) IN > > ((value1_0,value2_0),(value1_1,value2_1) ...) > > > > (which is legal on oracle 8i on solaris 8). > > I know this has an equivalent boolean expression but: > > -I would like to use an index defined like > > CREATE INDEX myindex ON table(attribute1,attribute2) > > -the pairs in the list can be many (up to 100) > > or'ing IN terms together is probably going to result in the same plan that > you are describing above (assuming table has a large enough number > of rows). I don't know if this way of writing the query can generate > a sort of values you are checking against and then a merge join. > For just a hundred or so values I don't think this plan would be that > much better than both nest loop and multiple index scans. However, if > you want to get that plan and or'ing INs won't generate it, then you could > try union'ing the values together in the IN value list. 7.4 will likely > behave differently than pre 7.4 versions. Thanks for the answer, it works (i.e. the planner uses the index). Is the query limited in length (in characters)?Marco -- Marco Vezzoli tel. +39 039 603 6852 STMicroelectronics fax. +39 039 603 5055