Re: Index only scan

On 11/10/12 12:41, Tom Lane wrote:
> Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> writes:
>> On 11/10/12 01:03, Lars Helge Øverland 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
Thanks for the correction!

Cheers,
Gavin