On 11/10/12 12:41, Tom Lane wrote:
Gavin Flower <GavinFlower@xxxxxxxxxxxxxxxxx> 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
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