Re: [HACKERS] WIP: Aggregation push-down

On Fri, Feb 23, 2018 at 11:08 AM, Antonin Houska <ah@cybertec.at> wrote:
> I spent some more time thinking about this. What about adding a new strategy
> number for hash index operator classes, e.g. HTBinaryEqualStrategyNumber? For
> most types both HTEqualStrategyNumber and HTBinaryEqualStrategyNumber strategy
> would point to the same operator, but types like numeric would naturally have
> them different.
>
> Thus the pushed-down partial aggregation can only use the
> HTBinaryEqualStrategyNumber's operator to compare grouping expressions. In the
> initial version (until we have useful statistics for the binary values) we can
> avoid the aggregation push-down if the grouping expression output type has the
> two strategies implemented using different functions because, as you noted
> upthread, grouping based on binary equality can result in excessive number of
> groups.
>
> One open question is whether the binary equality operator needs a separate
> operator class or not. If an opclass cares only about the binary equality, its
> hash function(s) can be a lot simpler.

Hmm.  How about instead adding another regproc field to pg_type which
stores the OID of a function that tests binary equality for that
datatype?  If that happens to be equal to the OID you got from the
opclass, then you're all set.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company