Re: Planner producing 100% duplicate subplans when unneeded

On Mon, Sep 27, 2010 at 5:09 PM, Daniel Grace <dgrace(at)wingsnw(dot)com> wrote:
> Is there any chance this might be looked at in a future release?

This is another interesting example of a case where an inlining-type
optimization (which is effectively what's happening here, I think)
turns out to be a negative. We had one a while back that involved
actual function inlining, which is not quite what's happening here,
but it's close. It doesn't seem too hard to figure out whether or not
inlining is a win (non-trivial subexpressions should probably never
get duplicated), but nobody's gotten around to writing the logic to
make it work yet. One useful technique is to stick "OFFSET 0" into
the subquery; that prevents it from being inlined and gives you
something more like the plan you were hoping for.

Whether or not this will get looked at in a future release is a tough
question to answer. It's possible that someone (most likely, Tom)
will get motivated to fix this out of sheer annoyance with the current
behavior, or will notice a way to improve it incidentally while making
some other change. But of course the only way to make sure it gets
fixed is to do it yourself (or pay someone to do it).

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company