Re: SP-GiST versus index-only scans

Jesper Krogh <jesper@krogh.cc> writes:
> On 2011-12-14 19:48, Tom Lane wrote:
>> I think this is somewhat wishful thinking unfortunately.  The difficulty
>> is that if the index isn't capable of reconstructing the original value,
>> then it's probably giving only an approximate (lossy) answer, which
>> means we'll have to visit the heap to recheck each result, which
>> pretty much defeats the purpose of an index-only scan.

> I can see that it is hard to generalize, but in the tsvector case the
> we are indeed not capable of reconstructing the row since the
> positions are not stored in the index, the actual lookup is not a
> lossy and I'm fairly sure (based on experience) that pg dont
> revisit heap-tuples for checking (only for visibillity).

Well, the way the tsvector code handles this stuff is that it reports
the result as lossy only if the query actually poses a constraint on
position (some do, some don't).  That case was actually what made us
move the determination of lossiness from plan time to execution time,
since in the case of a non-constant tsquery, there's no way for the
planner to know about it (and even with the constant case, you'd need a
helper function that doesn't exist today).  But this behavior is
problematic for index-only scans because the planner can't tell whether
a query will be lossy or not, and it makes a heck of a lot bigger
difference than it used to.
        regards, tom lane