SELECT statement never completes. - Mailing list pgsql-sql

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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