How could a predicate be pushable to only one side of a union? Can
you provide an example of a predicate that can be pushed only to one side?
If we take something along the lines of:
select ... from
t2,
(select * from t1 union values (1,2), (3,4), (5,6)) X1 (a,b)
where X1.a = t2.i;
In this case the predicate X1.a = t2.i could be pushed to the left
("select * from t1") and used when reading T1, but couldn't be
pushed to the VALUES clause because there's no underlying table.
OK. One way to deal with this is to put a ProjectRestrictNode between
the union node and the values clause as a place to park the
predicate. To make things simple, you might want to always put
ProjectRestrictNodes under both sides of the union during
preprocessing (i.e. after binding but before optimization). In some
cases the extra nodes won't be needed, but ProjectRestrictNodes (and
the corresponding ProjectRestrictResultSets) are cheap. Also, you
could eliminate unnecessary ProjectRestrictNodes at the end of
optimization (possibly in modifyAccessPaths()).
This approach would give better performance in some cases, and could
simplify the code (since you wouldn't have to figure out when the
predicates are pushable).
- Jeff Lichtman
[EMAIL PROTECTED]
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