Ehab Galal <ehabgalal123@hotmail.com> writes:
> explain select *
> from t1, t2, t3
> where t1.f <= t2.f
> and t2.f <= t3.f
> and t1.f <= t3.f;
> I was wondering if there is a
> way to omit the redundant join predicate.
You're not being very clear here. Do you mean will you get the same
answer if you omit "t1.f <= t3.f"? Yes, of course (ignoring possibly
different output ordering). Do you mean you think the system should
discard it as redundant? I disagree --- the more join clauses the
better, as a rule. Do you mean that the EXPLAIN output looks like
the same comparison is being applied twice? It isn't --- in a more
modern PG release the output looks like this:
QUERY PLAN
------------------------------------------------------------------Nested Loop (cost=33.54..81794021.44 rows=362975624
width=12) Join Filter: ((t1.f <= t2.f) AND (t2.f <= t3.f)) -> Nested Loop (cost=0.00..124472.40 rows=1526533
width=8) Join Filter: (t1.f <= t3.f) -> Seq Scan on t1 (cost=0.00..31.40 rows=2140 width=4) ->
SeqScan on t3 (cost=0.00..31.40 rows=2140 width=4) -> Materialize (cost=33.54..54.94 rows=2140 width=4) ->
SeqScan on t2 (cost=0.00..31.40 rows=2140 width=4)
(8 rows)
This is of course the stupidest possible join plan, but it's hard to do
much better --- both hash and merge joins work only on equality
conditions. You can do a bit better with an index on t2.f:
QUERY PLAN
----------------------------------------------------------------------Nested Loop (cost=0.00..13222230.60
rows=362975624width=12) -> Nested Loop (cost=0.00..124472.40 rows=1526533 width=8) Join Filter: (t1.f <=
t3.f) -> Seq Scan on t1 (cost=0.00..31.40 rows=2140 width=4) -> Seq Scan on t3 (cost=0.00..31.40
rows=2140width=4) -> Index Scan using t2i on t2 (cost=0.00..5.01 rows=238 width=4) Index Cond: ((t1.f <=
t2.f)AND (t2.f <= t3.f))
(7 rows)
regards, tom lane
