Maybe someone can explain this to me.

When I do an UPDATE with a sub SELECT, the Query Plan shows a table
scan.

However, if I change the IN clause to use explicit values (say some
comma delimited integers), it uses my index.

Something like:

UPDATE Foo
SET val=0
WHERE id IN ( 1,2,3,4 );

This shows the query using the index on Foo.id.

UPDATE Foo
SET val=0
WHERE id IN ( SELECT Bar.id FROM Bar );

This shows the query performing a table scan on Foo.

What gives? I'm using IBExpert to determine the Query Plan.

R.