Re: strange view performance

Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> I am solving a strange situation, where using a view is slower than
> using same tables directly.

> The view is defined as

> CREATE VIEW v1 AS
> SELECT *
> FROM A
> LEFT JOIN B
> LEFT JOIN C
> LEFT JOIN D

> and query is
> SELECT *
> FROM T
> LEFT JOIN v

> this query is slower than:

> SELECT *
> FROM T
> LEFT JOIN A
> LEFT JOIN B
> LEFT JOIN C
> LEFT JOIN D

> Is there a some reason for this behave?

Well, they don't necessarily mean the same thing --- these are only
logically equivalent if the left joins all commute, which would depend
on the ON conditions.

> [ EXPLAIN outputs ]

But I also notice that you are using collapse/geqo limits of 12 for
queries that involve 13 base relations, so that'd mean that syntactic
differences could lead to plan differences too.

regards, tom lane