Re: optimizer, view, union

Markus Bertheau <twanger(at)bluetwanger(dot)de> writes:
> Can pg transform

> SELECT * FROM (
> SELECT 'foo' AS class, id FROM foo
> UNION ALL
> SELECT 'bar' AS class, id FROM bar
> ) AS a WHERE class = 'foo'

[ experiments... ] Yes, if you spell it like this:

regression=# explain SELECT * FROM (
regression(# SELECT 'foo'::text AS class, id FROM foo
regression(# UNION ALL
regression(# SELECT 'bar'::text AS class, id FROM bar
regression(# ) AS a WHERE class = 'foo';
QUERY PLAN
-------------------------------------------------------------------------
Append (cost=0.00..105.60 rows=4280 width=4)
-> Subquery Scan "*SELECT* 1" (cost=0.00..52.80 rows=2140 width=4)
-> Seq Scan on foo (cost=0.00..31.40 rows=2140 width=4)
-> Subquery Scan "*SELECT* 2" (cost=0.00..52.80 rows=2140 width=4)
-> Result (cost=0.00..31.40 rows=2140 width=4)
One-Time Filter: false
-> Seq Scan on bar (cost=0.00..31.40 rows=2140 width=4)
(7 rows)

If unadorned, the literals get caught up in some type-conversion issues.
(You don't really want them in the output of a view anyway; "unknown"
type columns are bad news.)

regards, tom lane