Re: Joining on a view containing a UNION ALL produces a suboptimal plan on 9.3.4

Chris Hanks <christopher(dot)m(dot)hanks(at)gmail(dot)com> writes:
> CREATE VIEW tables AS
> SELECT a.*, b.col AS other_col
> FROM a
> LEFT JOIN b ON a.id = b.id
> UNION ALL
> SELECT c.*, d.col AS other_col
> FROM c
> LEFT JOIN d ON c.id = d.id;

> EXPLAIN ANALYZE
> SELECT *
> FROM tables
> WHERE id = 89; -- Index scans, as expected.

> EXPLAIN ANALYZE
> SELECT *
> FROM e
> JOIN tables ON e.col = tables.id
> WHERE e.id = 568; -- Big merge joins, when simple index scans should
> be possible?

> Would this be considered a deficiency in the optimizer? Is there a simple fix?

Don't hold your breath. To arrive at the
union-on-the-inside-of-a-nestloop plan you're hoping for, the planner
would have to create a "parameterized path" for the UNION ALL structure.
But when you have joins in the arms of the UNION ALL, they are considered
to be independent subqueries, and we currently have a policy decision not
to try to generate parameterized paths for subqueries. It'd be quite
expensive and I think the planner is probably lacking some necessary
mechanisms anyway.

Given that e.id is unique, you could possibly fake it with something like

select * from tables where id = (select e.col from e where e.id = 568);

regards, tom lane