Re: [HACKERS] path toward faster partition pruning

Thanks for the comment.

On 2018/04/10 21:11, Ashutosh Bapat wrote:
> On Tue, Apr 10, 2018 at 5:32 PM, David Rowley
> <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
>> Apart from that confusion, looking at the patch:
>>
>> +CREATE OR REPLACE FUNCTION pp_hashint4_noop(int4, int8) RETURNS int8 AS
>> +$$SELECT coalesce($1)::int8$$ LANGUAGE sql IMMUTABLE STRICT;
>> +CREATE OPERATOR CLASS pp_test_int4_ops FOR TYPE int4 USING HASH AS
>> +OPERATOR 1 = , FUNCTION 2 pp_hashint4_noop(int4, int8);
>> +CREATE OR REPLACE FUNCTION pp_hashtext_length(text, int8) RETURNS int8 AS
>> +$$SELECT length(coalesce($1))::int8$$ LANGUAGE sql IMMUTABLE STRICT;
>>
>>
>> Why coalesce here? Maybe I've not thought of something, but coalesce
>> only seems useful to me if there's > 1 argument. Plus the function is
>> strict, so not sure it's really doing even if you added a default.
>
> I think Amit Langote wanted to write coalesce($1, $2), $2 being the
> seed for hash function. See how hash operator class functions are
> defined in sql/insert.sql.

Actually, I referenced functions and operator classes defined in
hash_part.sql, not insert.sql. Although as you point out, I didn't think
very hard about the significance of $2 passed to coalesce in those
functions. I will fix that and add it back, along with some other changes
that makes them almost identical with definitions in hash_part.sql.

> May be we should just use the same
> functions or even the same tables.

Because hash_part.sql and partition_prune.sql tests run in parallel, I've
decided to rename the functions, operator classes, and the tables in
partition_prune.sql. It seems like a good idea in any case. Also, since
the existing pruning tests were written with that table, I decided not to
change that.

Will post an updated patch after addressing David's comment.

Regards,
Amit