Zdenek Kotala <Zdenek.Kotala@Sun.COM> writes:
> I think it depends of ration of unique integer number in a table and
> numbers of requested interval, number distribution and total number of rows.
> For example if you have 10 distinct number and each has 100 occurrence
> then full scan is better (for between 1 and 5). But if each number
> occurs 100000x. Then using hash index should be effective.
I think this discussion is a complete waste of time. Hash indexes don't
win against btrees for single indexscans currently. Even if that ever
gets fixed, it's highly unlikely that they'd win for N separate
indexscans versus 1 indexscan, which is what a query rewrite of this
sort would produce. Remember that the btree will have the desired range
of keys stored adjacently, whereas in a hash they are almost certainly
in distinct buckets, and likely not even close-together buckets if the
hash function is doing its job well. So you really are talking about a
factor of N both in indexscan setup overhead and in I/O costs.
regards, tom lane
