Обсуждение: json_populate_recordset

 From here:

https://www.postgresql.org/docs/10/static/functions-json.html

select * from json_populate_record(null::myrowtype, '{"a": 1, "b": ["2", 
"a b"], "c": {"d": 4, "e": "a b c"}}')

What is the null::myrowtype doing?


Thanks,
-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 06/07/2018 09:56 AM, Pavel Stehule wrote:
> 
> 
> 2018-06-07 18:51 GMT+02:00 Adrian Klaver <adrian.klaver@aklaver.com 
> <mailto:adrian.klaver@aklaver.com>>:
> 
>      From here:
> 
>     https://www.postgresql.org/docs/10/static/functions-json.html
>     <https://www.postgresql.org/docs/10/static/functions-json.html>
> 
>     select * from json_populate_record(null::myrowtype, '{"a": 1, "b":
>     ["2", "a b"], "c": {"d": 4, "e": "a b c"}}')
> 
>     What is the null::myrowtype doing?
> 
> 
> this function uses polymorphic parameters - its some like
> 
> create or replace function fx(in anyelement, json) returns anyelement ..
> 
> output polymorphic value requires minimally one input polymorphic value 
> for type specification. Postgres has not possibility to pass only 
> datatype - so pattern null:type is used instead.

Alright, so it is bootstrapping a polymorphic type off null. Thanks.

> 
> Regards
> 
> Pavel
> 
> 
> 
>     Thanks,
>     -- 
>     Adrian Klaver
>     adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com