PostgreSQL函数：将多行返回为JSON

Question

I have a PostgreSQL function which successfully returns its data as

table(key character varying, value integer)

but I want to return JSON as it's more convenitent. I have looked at some other answers, and documentation, but it gets complicated when aliases are used (I'd like to return 'key' and 'value' as the column names).

Can someone please recommend the most simple and concise way of expressing this function, in a way that does not require extra complexity to call:

DROP FUNCTION get_document_metadata_integer_records_by_document_id(integer);

CREATE OR REPLACE FUNCTION get_document_metadata_integer_records_by_document_id(document_id integer)
  RETURNS table(key character varying, value integer) AS
$BODY$

  SELECT document_metadata_records.key, document_metadata_integer_records.value FROM document_metadata_integer_records
  LEFT OUTER JOIN document_metadata_records
  ON document_metadata_integer_records.document_metadata_record_id = document_metadata_records.id
  WHERE document_metadata_records.document_id = $1

$BODY$
  LANGUAGE sql VOLATILE
  COST 100;
ALTER FUNCTION get_document_metadata_integer_records_by_document_id(integer)
  OWNER TO postgres;

Answer 1

Use cross join lateral as a clean and easy path to create a composite type that can be used by row_to_json

create or replace function get_document_metadata_integer_records_by_document_id(
    _document_id integer
) returns setof json as $body$

select row_to_json(s)
from
    document_metadata_integer_records dmir
    left outer join
    document_metadata_records dmr on dmir.document_metadata_record_id = dmr.id
    cross join lateral
    (select dmr.key, dmir.value) s
where dmr.document_id = _document_id
;
$body$ language sql stable;

then use it as

select get_document_metadata_integer_records_by_document_id(1) as pair;

or

select pair from get_document_metadata_integer_records_by_document_id(1) j(pair);

or make it return a table

) returns table (pair json) as $body$

and use as

select pair from get_document_metadata_integer_records_by_document_id(1);

With older versions of Postgresql without lateral it is possible to do it in a subquery

select row_to_json(s)
from
    (
        select dmr.key, dmir.value
        from 
            document_metadata_integer_records dmir
            left outer join
            document_metadata_records dmr on dmir.document_metadata_record_id = dmr.id
        where dmr.document_id = _document_id
    ) s