Thread: A pessimistic planner

Another day, another timing out query rewritten to force a more stable
query plan.

While I know that the planner almost always chooses a good plan, I
tend to think it is trying too hard. While 99% of the queries might be
10% faster, 1% might be timing out which makes my users cross and my
life difficult. I'd much rather have systems that are less efficient
overall, but stable with a very low rate of timeouts.

I was wondering if the planner should be much more pessimistic,
trusting in Murphy's Law and assuming the worst case is the likely
case? Would this give me a much more consistent system? Would it
consistently grind to a halt doing full table scans? Do we actually
know the worst cases, and would it be a relatively easy task to update
the planner so we can optionally enable this behavior per transaction
or across a system? Boolean choice between pessimistic or optimistic,
or is pessimism a dial?

--
Stuart Bishop <stuart@stuartbishop.net>
http://www.stuartbishop.net/

Stuart Bishop <stuart@stuartbishop.net> writes:
> I was wondering if the planner should be much more pessimistic,
> trusting in Murphy's Law and assuming the worst case is the likely
> case? Would this give me a much more consistent system?

Well, it would give uniformly awful plans for cases where fast-start
plans are actually useful ... and there are plenty.

We do need to think of ways to account for risk, but I don't think
that jamming the tiller all the way over is likely to be a net
improvement.

            regards, tom lane