On 3/30/15 10:52 AM, Tom Lane wrote:
> Kevin Grittner<kgrittn@ymail.com> writes:
>> >Tom Lane<tgl@sss.pgh.pa.us> wrote:
>>> >>But the other problem is that the planner considers less-than-1%
>>> >>differences in cost estimates to be "in the noise", which means
>>> >>that it's not going to consider cost differences of less than
>>> >>1480 units in the remaining join steps to be significant. This
>>> >>is how come we end up with the apparently brain-dead decisions to
>>> >>use seqscans on some of the other tables such as "pi" and "ac":
>>> >>comparing the seqscan to a potential inner indexscan, the total
>>> >>cost of the join is "the same" according to the 1% rule,
>> >The 1% rule itself might be something to add to the R&D list.
> Perhaps. But it does make for a significant difference in planner speed,
> and I would argue that any case where it really hurts is by definition
> a cost estimation failure somewhere else.
What I wish we had was some way to represent "confidence" in the
accuracy of a specific plan node, with the goal of avoiding plans that
cost out slightly cheaper but if we guessed wrong on something will blow
up spectacularly. Nested loops are an example; if you miscalculate
either of the sides by very much you can end up with a real mess unless
the rowcounts were already pretty trivial to begin with.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com
