> On Thu, 13 Sep 2018 at 21:36, Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru> wrote:
>
> El 13/09/18 a las 18:39, Jesper Pedersen escribió:
>
>> I think we can improve this,
>> and the skip scan can be strictly faster than index scan regardless of
>> the data. As a first approximation, imagine that we somehow skipped
>> equal tuples inside _bt_next instead of sending them to the parent
>> Unique node. This would already be marginally faster than Unique + Index
>> scan. A more practical implementation would be to remember our position
>> in tree (that is, BTStack returned by _bt_search) and use it to skip
>> pages in bulk. This looks straightforward to implement for a tree that
>> does not change, but I'm not sure how to make it work with concurrent
>> modifications. Still, this looks a worthwhile direction to me, because
>> if we have a strictly faster skip scan, we can just use it always and
>> not worry about our unreliable statistics. What do you think?
>>
>
> This is something to look at -- maybe there is a way to use
> btpo_next/btpo_prev instead/too in order to speed things up. Atm we just
> have the scan key in BTScanOpaqueData. I'll take a look after my
> upcoming vacation; feel free to contribute those changes in the meantime
> of course.
But having this logic inside _bt_next means that it will make a non-skip index
only scan a bit slower, am I right? Probably it would be easier and more
straightforward to go with the idea of dynamic fallback then. The first naive
implementation that I came up with is to wrap an index scan node into a unique,
and remember estimated number of groups into IndexOnlyScanState, so that we can
check if we performed much more skips than expected. With this approach index
skip scan will work a bit slower than in the original patch in case if
ndistinct is correct (because a unique node will recheck rows we returned), and
fallback to unique + index only scan in case if planner has underestimated
ndistinct.