Once again, a slightly convoluted question, but it seems that PG may be
doing a little more work than is necessary when selecting from views with
sub-selects. It seems that every time a view field is being referenced in
an outer select expression, the view field is being re-evaluated. Is there
any way to get PG to know that it only needs to do the aggregate once?
eg.
create table b(f1 int, f2 int); create table r(f1 int);
create view bv as select f1,f2, exists(select * from r where r.f1=b.f1) as has_f1, exists(select * from r
wherer.f1=b.f2) as has_f2 from b;
explain select f1,f2, case when has_f1 and has_f2 then 'both' when has_f1 then 'f1_only'
when has_f2 then 'f2_only' else 'none' end as status from bv;
Seq Scan on b (cost=0.00..20.00 rows=1000 width=8) SubPlan -> Seq Scan on r (cost=0.00..22.50 rows=5
width=4) -> Seq Scan on r (cost=0.00..22.50 rows=5 width=4) -> Seq Scan on r (cost=0.00..22.50 rows=5
width=4) -> Seq Scan on r (cost=0.00..22.50 rows=5 width=4)
----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.B.N. 75 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 0500 83 82 82 | ___________ |
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PGP key available upon request, | /
and from pgp5.ai.mit.edu:11371 |/
