Re: BUG #4748: hash join and sort-merge join make different results

On 2009-04-03 23:32 Tom Lane said the following:
> Roman Kononov <kononov@ftml.net> writes:
>> On 2009-04-03 14:57 Tom Lane said the following:
>>> I think we could fix this by having interval_hash() duplicate the
>>> total-span calculation done by interval_cmp_internal, and then return
>>> the hash of the resulting TimeOffset.  This is going to break existing
>>> hash indexes on intervals, but there seems little choice...
>
>> Consider hashing the result of justify_interval().
>
> Uh, what's your point?  We have to match interval_eq, not
> justify_interval.

For any two intervals a and b, saying that interval_cmp_interval(a,b)==0
is exactly the same as saying that (aj.month==bj.month && aj.day==bj.day
&& aj.time==bj.time), where aj=justify_interval(a) and
bj=justify_interval(b). Therefore, instead of hashing
interval_cmp_value() you can hash justify_interval(), where
interval_cmp_value() is the transformation of intervals in
interval_cmp_interval().

You said that hashing interval_cmp_value() breaks existing hash indexes.
Hashing "justified" intervals avoids such breaking in some cases.