Re: [PATCH] Equivalence Class Filters

On 12/6/15 10:38 AM, Tom Lane wrote:

I said "in most cases".  You can find example cases to support almost any
weird planner optimization no matter how expensive and single-purpose;
but that is the wrong way to think about it.  What you have to think about
is average cases, and in particular, not putting a drag on planning time
in cases where no benefit ensues.  We're not committing any patches that
give one uncommon case an 1100X speedup by penalizing every other query 10%,
or even 1%; especially not when there may be other ways to fix it.

This is a problem that seriously hurts Postgres in data warehousing applications. We can't keep ignoring optimizations that provide even as little as 10% execution improvements for 10x worse planner performance, because in a warehouse it's next to impossible for planning time to matter.

Obviously it'd be great if there was a fast, easy way to figure out whether a query would be expensive enough to go the whole 9 yards on planning it but at this point I suspect a simple GUC would be a big improvement.