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Message-ID: <c2d9e70e0509301140v115d3978wea086f463c6e3f3@mail.gmail.com>
Date: Fri, 30 Sep 2005 13:40:09 -0500
From: Jaime Casanova <systemguards@gmail.com>
Reply-To: Jaime Casanova <systemguards@gmail.com>
To: "jan aerts (RI)" <jan.aerts@bbsrc.ac.uk>
Subject: Re: combination of function to simple query makes query slow
Cc: pgsql-sql@postgresql.org
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References: 
 <84DA9D8AC9B05F4B889E7C70238CB45101FD6555@rie2ksrv1.ri.bbsrc.ac.uk>

On 9/30/05, jan aerts (RI) <jan.aerts@bbsrc.ac.uk> wrote:
> Hi,
>
> I'm having some problems in combining a function within a standard
> query.
>
> I basically have two tables (see between hashes below):
> * alias_table: objects can have different names (e.g. name1 is an alias
> of name2, and name3 is an alias of name4)
> * mapping_table: shows objects that 'lead to' other objects (e.g. the
> object with name1 leads to the object with name5)
>
> I want to traverse the mapping_table in a transparent way: if a first
> object leads to a second object, and that second object leads to a third
> object, I want to list the first and the third objects. So from the
> example table below: name6 leads to name2, which is an alias of name1,
> which in its turn leads to name5. The result of my query should be:
> name6 leads to name5.
>
> To accomplish this, I wrote a little function ('aliases_of'), as
> described below between the hashes.
>
> The query
>  SELECT aliases_of(1);
> returns:
>  aliases_of
>  ----------
>           1
>           2
> , which is the expected output.
>
> The query
>  SELECT * FROM mapping_table
>  WHERE object1_id IN ( SELECT aliases_of(2) );
> returns:
>  object1_id | object1_name | object2_id | object2_name
>  -----------------------------------------------------
>  1          | name1        | 5          | name5
> , which is the expected output, because it is equivalent to:
>  SELECT * FROM mapping_table
>  WHERE object1_id IN ( 1, 2 );
>
> However, the query that would traverse the data:
>  SELECT m1.object1_id, m1.object1_name, m2.object2_id, m2.object2_name
>  FROM mapping_table m1, mapping_table m2
>  WHERE m1.object2_id IN ( SELECT aliases_of(m2.object1_id) );
> just keeps on running, and never seems to stop. (I must say that that
> table in effect contains about a million records.)
> The expected output would have been:
>  object1_id | object1_name | object2_id | object2_name
>  -----------------------------------------------------
>  6          | name6        | 5          | name5
>
> I also tried to following, which doesn't work either.
>  SELECT m1.object1_id, m1.object1_name, m2.object2_id, m2.object2_name
>  FROM mapping_table m1 JOIN mapping_table m2 ON ( m1.object2_id IN (
> SELECT aliases_of(m2.object1_id) );
>
> Is there a way to speed up this query? I'd also have to be able to
> traverse the mapping_table ever further, following a path of things that
> 'lead to' other things.
>
> I hope I made myself clear a bit...
> Any help appreciated. Many thanks,
> jan.
>
> ################################################################
> (A) TABLE: alias_table
>  object1_id | object1_name | object2_id | object2_name
>  -----------------------------------------------------
>  1          | name1        | 2          | name2
>  3          | name3        | 4          | name4
>
> (B) TABLE: mapping_table
>  object1_id | object1_name | object2_id | object2_name
>  -----------------------------------------------------
>  1          | name1        | 5          | name5
>  6          | name6        | 2          | name2
>
> (C) FUNCTION: aliases_of
>  CREATE FUNCTION aliases_of(bigint) RETURNS SETOF bigint
>  AS 'SELECT $1
>      UNION
>      SELECT object1_id FROM alias_table WHERE object2_id =3D $1
>      UNION
>      SELECT object2_id FROM alias_table WHERE object1_id =3D $1
>     '
>  LANGUAGE SQL;
> ################################################################
>
>


maybe you can show us an EXPLAIN  of your select:

EXPLAIN select_statement


--
Atentamente,
Jaime Casanova
(DBA: DataBase Aniquilator ;)