Re: Index only scan

Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> writes:
> On 11/10/12 01:03, Lars Helge verland wrote:
>> My question is: Would it be feasible and/or possible to implement
>> index only scans in a way that it could take advantage of several,
>> single-column indexes? For example, a query spanning columns a, b, c
>> could take advantage of 3 single-column indexes put on columns a, b,
>> c.

> Index only scans do use multiple indexes of single fields where
> appropriate. Here the planner determined it only needed to scan 2 of
> the 3 relevant single field indexes.

But your example isn't an index-only scan ... it's a plain old bitmap
scan, and so it does touch the heap.

The difficulty with what Lars proposes is that there's no way to scan
the different indexes "in sync" --- each one will be ordered according
to its own column order. In principle I guess we could read out the
index data and do a join using the ctid's, but it's far from clear that
such a thing would be worth the trouble.

regards, tom lane