Re: Hashable custom types

Greg Stark <stark@mit.edu> writes:
> On Sat, Apr 26, 2014 at 6:39 PM, Atri Sharma <atri.jiit@gmail.com> wrote:
>> Without sorting, isnt the scope of a recursive UNION with custom datatypes
>> pretty restrictive?

> All the default data types are hashable. It's not hard to add a hash
> operator class. In a clean slate design it would probably have been
> simpler to just make it a requirement that any data type provide a
> default hash operator (and probably a default btree comparator).
> Postgres provides a lot of degrees of freedom but it should probably
> be considered best practice to just provide both even if you don't
> envision one or the other being used directly by users for indexes.

A btree opclass requires that you invent some one-dimensional sort order
for the datatype, which might be a difficult thing; so I think it's fully
reasonable not to require datatypes to have btree support.  Hashing
doesn't require any semantic assumptions beyond having an equality rule,
which is clearly *necessary* if you want to do stuff like UNION or
DISTINCT.  So from that standpoint it's perfectly reasonable for recursive
UNION to require a hashable equality operator, whereas the other case of
requiring a sortable operator would be a lot harder to defend.

Having said that, I can also believe that there might be datatypes for
which implementing a hash function would be a lot harder than implementing
sorting; this could be true if your equality rule allows for a lot of
different physical representations of "equal" values.  But I'm not so
excited about such cases that I want to do the work of figuring out a
way to implement recursive UNION by sorting.
        regards, tom lane