Thread: SELECT statement never completes.

pgsql-sql,

We are porting a database from IBM DB2 to PostgreSQL. 
In several related scripts, there is a SELECT
statement that never completes in Postgres but
completes in a few seconds using DB2, for example:

Table row count:
SELECT count(*) FROM tableX;112671
SELECT count(*) from tableY;314625

This statement does not complete:
SELECT id FROM tableX WHERE id NOT IN (SELECT id FROM
tableY);

Any suggestions?
Thanks

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On Wed, 9 Oct 2002, John Pauley wrote:

> pgsql-sql,
>
> We are porting a database from IBM DB2 to PostgreSQL.
> In several related scripts, there is a SELECT
> statement that never completes in Postgres but
> completes in a few seconds using DB2, for example:
>
> Table row count:
> SELECT count(*) FROM tableX;
>  112671
> SELECT count(*) from tableY;
>  314625
>
> This statement does not complete:
> SELECT id FROM tableX WHERE id NOT IN (SELECT id FROM
> tableY);
>
> Any suggestions?

Unfortunately IN <subselect> tends to have poor performance
in postgresql.  Often you can get better performance out
of exists, but not always.
You might want to try:
select id from tableX WHERE NOT EXISTS (select * fromtableY where tableY.id=tableX.id);
and see if it runs better.