Postgresql – “ERROR:malformed array literal” when using json_to_record with a JSON array element in Postgres 9.4

jsonpostgresqlpostgresql-9.4

This illustrates the issue nicely:

When column b is of type text, and not an array, the following works:

select * 
from json_to_record('{"a":1,"b":["hello", "There"],"c":"bar"}') 
    as x(a int, b text, d text);

 a |         b          | d
---+--------------------+---
 1 | ["hello", "There"] |

But if I define the b column as an array, I get this error:

select * 
from json_to_record('{"a":1,"b":["hello", "There"],"c":"bar"}') 
    as x(a int, b text[], d text)

ERROR:  malformed array literal: "["hello", "There"]"
DETAIL:  "[" must introduce explicitly-specified array dimensions.

How can I convince/coerce json_to_record (or json_populate_record) to convert a JSON array into the Postgres array of the target column type?

Best Answer

Just a slight variation to Chris's answer:

SELECT a, translate(b, '[]', '{}')::text[] AS b, d
FROM json_to_record('{"a": 1, "b": ["hello", "There"], "c": "bar"}')
AS x(a int, b text, d text);

The idea is the same: massage the JSON array into an array - in this case, through an array literal. In addition to a bit cleaner looking code (though I love it, regex usually does not help much in this regard :), it seems slighly faster, too:

CREATE TABLE jsonb_test (
    id serial,
    data jsonb
);

INSERT INTO jsonb_test (id, data)
SELECT i, format('{"a": %s, "b": ["foo", "bar"], "c": "baz"}', i::text)::jsonb 
FROM generate_series(1,10000) t(i);

SELECT a, string_to_array(regexp_replace(b, '\[*\"*\s*\]*','','g'),',') AS b, d
FROM jsonb_test AS j, 
LATERAL json_to_record(j.data::json) AS r(a int, b text, d text);

-- versus 

SELECT a, translate(b, '[]', '{}')::text[] AS b, d
FROM jsonb_test AS j, 
LATERAL json_to_record(j.data::json) AS r(a int, b text, d text);

On this dataset and on my test box, the regex version shows and average execution time of 300 ms, while my version shows 210 ms.

Related Question