2009/10/12 Teodor Sigaev <teodor@sigaev.ru>:
>> Are you planning to submit this as a /contrib module?
>
> I haven't objections to do that, we don't planned that because we know
> sceptical
> relation of community to hints :)
I think it would be pretty useful to have some additional knobs to
poke at the planner. I sometimes want to know what the planner thinks
the cost of some plan other than the one actually selected would be.
For simple queries, it's often possible to accomplish this by using
the enable_* parameters, but those are a pretty coarse instrument and
what you can do with them is fairly limited. So I think it would be
nice to have some more options, and I wouldn't object to including
this as one of them, provided that the code isn't too much of a
kludge.
That having been said, my tables don't tend to be heavily indexed and
the planner basically never picks the wrong one. Most of my query
planning problems (and many of the ones on -performance) are the
result of bad selectivity estimates. So what I'd really like to see
is a way to override the selectivity of a given expression. Making
the planner smarter about estimating selectivity in the first place
would be *great*, too, but I don't have much hope that it's ever going
to be perfect.
...Robert