Re: Is the subtype_diff function in CREATE TYPE only can be C function?

> 2024年1月10日 18:04，Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> 写道：
>
> On Wed, Jan 10, 2024 at 1:49 PM ddme <ret2ddme(at)qq(dot)com <mailto:ret2ddme(at)qq(dot)com>> wrote:
>>
>> Hi all,
>>
>> I notice that the CREATE TYPE syntax can specify subtype_diff function
>>
>> CREATE TYPE name AS RANGE (
>> SUBTYPE = subtype
>> [ , SUBTYPE_OPCLASS = subtype_operator_class ]
>> [ , COLLATION = collation ]
>> [ , CANONICAL = canonical_function ]
>> [ , SUBTYPE_DIFF = subtype_diff_function ] <————— here
>> [ , MULTIRANGE_TYPE_NAME = multirange_type_name ]
>> )
>>
>> And a example is
>> ```sql
>>
>> CREATE TYPE float8_range AS RANGE (subtype = float8, subtype_diff = float8mi);
>>
>> ```
>>
>> I notice that float8mi is a C function, and I find the call_subtype_diff() in source code that it seems only can call C function.
>
> call_subtype_diff() invokes FunctionCall2Coll() which in turn invokes
> the function handler for non-C functions. See
> fmgr_info_cxt_security() for example. So subtype_diff can be a SQL
> callable function written in any supported language.
>
>>
>> I want to know
>>
>> 1. Can the subtype_diff function in CREATE TYPE be sql or plpgsql function?
>
> I think so.
>
>> 2. How to call subtype_diff function? I know it related with GiST index, I need a example on how to trigger subtype_diff function.
>
> I am not familiar with GiST code enough to answer that question. But
> looking at the places where call_subtype_diff() is called, esp. the
> comments there might give you hints.
> OR somebody more familiar with GiST code will give you a direct answer.
>
> --
> Best Wishes,
> Ashutosh Bapat

Thank you!

I know that range_gist_picksplit call call_subtype_diff() but I find not call path for range_gist_picksplit.
I have try to trigger GiST index like `CREATE INDEX … USING GIST` and using select with filter to trigger index. With the help of EXPLAIN, I get that the gist index have been triggered but subtype_diff function have not

```sql
create function float4mi(a float8, b float8) RETURNS float8 LANGUAGE SQL … …

create type float8range as range (subtype=float8, subtype_diff=float4mi);
create table float8range_test(f8r float8range);
insert into float8range_test values('[1.111,2.344]'::float8range), ('[1.111, 4.567]'::float8range);
create index my_index on float8range_test using gist(f8r);
SET enable_seqscan = off;
select * from float8range_test ORDER BY f8r;
```

Is there need more setup SQL like `CREATE OPERATOR CLASS … USING gist` to trigger?