Tom Lane wrote:
> I've been thinking about how to improve the performance of queries using
> "WHERE x IN (subselect)" and "WHERE x NOT IN (subselect)".
>
> In the existing implementation, the subquery result is rescanned to look
> for a match to x each time the WHERE clause is executed; this essentially
> makes it work like a nestloop join of the stupidest variety. (We do
> stick a Materialize node atop the subselect if it looks complicated, but
> that's not a big help in typical cases.)
>
> I've thought of three alternative implementations that would perform
> better in various scenarios. Each would be relatively simple to
> implement; the problem I'm having is figuring out how to get the planner
> to choose the best one. The alternatives are basically:
>
[abbreviated]
> 1. Add FROM item
> 2. Hash-based
> 3. Inner indexscan
> 4. the existing implementation
> The difficulty is that it's not clear how to choose one of these four
> ways, short of planning the *entire* query from scratch all four ways :-(.
> This seems pretty grim. Approaches #2 and #3 could be handled as local
> transformations of the WHERE clause, but we couldn't choose which to use
> very well if we don't yet know how many outer rows the WHERE clause will
> be executed for. Approach #1 is really planning a completely different
> query --- one with an extra FROM-item --- and there's not going to be
> all that much commonality in the computations, unless we restrict where
> the added FROM-item can be joined to the others, which'd more or less
> defeat the purpose.
>
> Anyone see a way around this difficulty?
How about starting with a rule-based method to make the choice?
1. If uncorrelated: use hash-based approach - ISTM this might address a large percentage of the problem cases -- it
couldeven handle the "IN (list-of-scalars)" case. Could it fall back to a tuplesort/binary-search for the too many
tohash in memory case?
2. If correlated: use an inner indexscan
3. If you come up with a pattern where none of the approaches produce a correct answer, use the existing
implementation
You could always get fancier later if needed, but something along these lines
would be a great start.
Joe
