review: Built-in binning functions

review: https://commitfest.postgresql.org/action/patch_view?id=1484

Hello

I did review of this patch, that add three functions varwidth_bucket for
types: anyelement, double and bigint

* This patch respects PostgreSQL coding rules
* it can applied without any issues
* there are no new compile warnings
* patch contains documentation and tests
* all tests was passed

* there was no any objection in related discussion. Functionality is clean
and based on current functionality of width_bucket.

My comments:

* I miss in documentation description of implementation - its is based on
binary searching, and when second parameter is unsorted array, then it
returns some nonsense without any warning.

* Description for anyelement is buggy twice times

"varwidth_bucket(5.35::numeric, ARRAY[1, 3, 4, 6]::numeric)"

probably should be "varwidth_bucket(5.35::numeric, ARRAY[1, 3, 4,
6]::numeric[])"

BUT it is converted to double precision, function with polymorphic
parameters is not used. So it not respects a widh_buckets model:

postgres=# \dfS width_bucket
List of functions
Schema │ Name │ Result data type │
Argument data types │ Type
────────────┼──────────────┼──────────────────┼───────────────────────────────────────────────────────────────┼────────
pg_catalog │ width_bucket │ integer │ double precision, double
precision, double precision, integer │ normal
pg_catalog │ width_bucket │ integer │ numeric, numeric, numeric,
integer │ normal
(2 rows)

There should be a interface for numeric type too. I am sure so important
part of code for polymorphic type can be shared.

Regards

Pavel