On Thu, Aug 17, 2006 at 09:21:33PM -0400, Tom Lane wrote:
> mark(at)mark(dot)mielke(dot)cc writes:
> > I have two simple queries that do what I believe to be the exact same
> > thing.
> These are actually not equivalent per spec.
> This still isn't equivalent to the join: it'll return at most one copy
> of any sm_change row, whereas you can get multiple copies of the same
> sm_change row from the join, if there were multiple matching sm_system
> rows. (Hm, given the unique index on (system_dbid, uid), I guess that
> couldn't actually happen --- but you have to reason about it knowing
> that that index is there, it's not obvious from the form of the query.)
> Anyway: given the way that the planner works, the IN form and the join
> form will probably take comparable amounts of time to plan. The "=
> subselect" form is much more constrained in terms of the number of
> alternative implementations we have, so it doesn't surprise me that it
> takes less time to plan.
That makes sense. Would it be reasonable for the planner to eliminate
plan considerations based on the existence of unique indexes, or is
this a fundamentally difficult thing to get right in the general case?
I did the elimination in my head, which is why I considered the plans to
be the same. Can the planner do it?
Sub-millisecond planning/execution for simple queries on moderate
hardware seems sexy... :-)
mark(at)mielke(dot)cc / markm(at)ncf(dot)ca / markm(at)nortel(dot)com __________________________
. . _ ._ . . .__ . . ._. .__ . . . .__ | Neighbourhood Coder
|\/| |_| |_| |/ |_ |\/| | |_ | |/ |_ |
| | | | | \ | \ |__ . | | .|. |__ |__ | \ |__ | Ottawa, Ontario, Canada
One ring to rule them all, one ring to find them, one ring to bring them all
and in the darkness bind them...
In response to
pgsql-performance by date
|Next:||From: mark||Date: 2006-08-18 02:30:32|
|Subject: Re: Q: Performance of join vs embedded query for simple queries?|
|Previous:||From: Tom Lane||Date: 2006-08-18 01:21:33|
|Subject: Re: Q: Performance of join vs embedded query for simple queries? |