On Feb 23, 2011, at 2:58 AM, Robert Haas wrote:
> 2011/2/22 PostgreSQL - Hans-Jürgen Schönig <postg...@cybertec.at>:
>> how does it work? we try to find suitable statistics for an arbitrary length
>> list of conditions so that the planner can use it directly rather than
>> multiplying all the selectivities. this should make estimates a lot more
>> precise.
>> the current approach can be extended to work with expressions and well as
>> "straight" conditions.
>
> /me prepares to go down in flames.
>
> Personally, I think the first thing we ought to do is add a real, bona
> fide planner hint to override the selectivity calculation manually,
> maybe something like this:
>
> WHERE (x < 5 AND y = 1) SELECTIVITY (0.1);
i thought there was an agreement that we don't want planner hints?
as tom pointed out - many broken queries come out of some query generator where
even the design to make the design is broken by design.
personally i like query generators as long as other people use them ... telling
people that this is the wrong way to go is actually financing my holiday next
week ... ;). in general - hibernate and stuff like that is a no-go.
personally i like the type of planner hints oleg and teodor came up with - i
think we should do more of those hooks they are using but hiding it in some
syntax is not a good idea.
it does not change the query and it still gives a lot of room to toy around. it
looks like a compromise.
however, oleg's contrib module does not fix the core problem of cross column
statistics because a hint is usually static but you want flexible selectivity.
regards,
hans
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