Re: To use a VIEW or not to use a View.....

On Wed, 22 Jan 2003, Tomasz Myrta wrote:

> >> Tomasz Myrta <jasiek(at)klaster(dot)net> writes:
> >> I'd like to split queries into views, but I can't join them - planner
> >> search all of records instead of using index. It works very slow.
>
>
> I think this is the same issue that Stephan identified in his response
> to your other posting ("sub-select with aggregate"). When you write
> FROM x join y using (col) WHERE x.col = const
> the WHERE-restriction is only applied to x. I'm afraid you'll need
> to write
> FROM x join y using (col) WHERE x.col = const AND y.col = const
> Ideally you should be able to write just
> FROM x join y using (col) WHERE col = const
> but I think that will be taken the same as "x.col = const" :-(

> I don't know if anything changed on 7.3.

I don't think so, but this is a general transitivity constraint AFAIK, not
one actually to do with views (ie, if you wrote out the query without a
view, you can run into the same issue). It's somewhat easier to run into
the case with views and the effect may be exasperated by views, but it's
a general condition.

For example:
create table a(a int);
create table c(a int);

sszabo=# explain select * from a join c using (a) where a=3;
QUERY PLAN
-------------------------------------------------------------
Hash Join (cost=1.01..26.08 rows=6 width=8)
Hash Cond: ("outer".a = "inner".a)
-> Seq Scan on c (cost=0.00..20.00 rows=1000 width=4)
-> Hash (cost=1.01..1.01 rows=1 width=4)
-> Seq Scan on a (cost=0.00..1.01 rows=1 width=4)
Filter: (a = 3)
(6 rows)

The filter is applied only to a. So, if you really wanted the
c.a=3 condition to be applied for whatever reason you're out of
luck.