Re: Factoring where clauses through UNIONS take 2

Stephan Szabo <sszabo(at)megazone23(dot)bigpanda(dot)com> writes:
> The thing that I think is killing it is the constants. A view like:
> create view qv1 as select a as b from q1 union select b from q2;
> explain select * from qv1 where b=3;
> pushes down into the selects.
> create view qv1 as select a as b, 'b' from q1 union select b,'c' from q2;
> explain select * from qv1 where b=3;
> doesn't.

Good catch. It would work the way Jonathan wants if he explicitly
coerces all those literals in the view definition to text (or *some*
specific datatype, anyway).

The reason the planner won't push down is that the result type of the
UNION's second column is TEXT, while the result types of the individual
sub-selects are UNKNOWN, and there are semantic issues with pushing down
a qual past a datatype conversion: it might not mean quite the same thing.
(Consider text vs char(n) and significant-blanks rules, for instance.)

In this particular case, since the qual doesn't actually touch the
type-converted column, it would have been safe to push down, but the
planner doesn't make this test on a per-qual basis: if there are type
conversions anywhere in the UNION it just punts.

regards, tom lane