Re: Planner avoidance of index only scans for partial indexes

Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
> If you create an index like this:
> create index on foo(a,b,v) where d = some_constant;
> there is no way to get an IOS on the index: you have to supply a the
> partial index exclusionary value to get the value of the index and
> that fools the IOS chooser because it doesn't see the value in the
> explicit list of index columns.

Yeah, this is a known limitation that we'll probably try to improve
someday. Per the comment in check_index_only():

/*
* Check that all needed attributes of the relation are available from the
* index.
*
* XXX this is overly conservative for partial indexes, since we will
* consider attributes involved in the index predicate as required even
* though the predicate won't need to be checked at runtime. (The same is
* true for attributes used only in index quals, if we are certain that
* the index is not lossy.) However, it would be quite expensive to
* determine that accurately at this point, so for now we take the easy
* way out.
*/

regards, tom lane