PostgreSQL – Convert Two Columns to JSON

jsonpostgresqlpostgresql-9.3

I have a json column with a json array in the form of

[{key : "key", value : "value"}, {key : "key2", value : "value2"}]

that I wish to convert to a new form

{"key" : "value", "key2" : "value2"}.

I can use the following query to get rows with key column and value column

SELECT json_array_elements(somejson)->'key', json_array_elements(somejson)->'value' FROM temp;

However I can't seem to come up with a way to convert that to a nice key-value JSON object. Since I'm on 9.3, I can't use json_object or json_build_object either.

Best Answer

Well, it would have been a lot faster and easier with 9.4, but it's doable with 9.3 in simple situations.

SELECT 
('{' ||       -- Opening brace for the JSON object
string_agg(   -- Get all the key/value pairs together in the same string
concat_ws(':', '"' || a || '"', b || ''), ',') || '}')::json -- Concatenate values to "key" : "value" format

FROM (
-- Extract the key and value from each array element
SELECT json_array_elements(myjson)->'key' a, 
json_array_elements(myjson)->'value' b FROM temp) AS FOO;

Not fun, but thankfully this is a one-time thing.

This can't be used to update a table, but you can create a temporary table and insert the values there first.

Related Solutions

PostgreSQL – Aggregate Objects into JSON Array

If you are on 9.4 something like this might be what you are after:

select json_object(array_agg(id)::text[],array_agg(rw)::text[])
from( select id
           , ( select to_json(array_agg(row_to_json(t)))
               from (select typ,prop from bgb where id=b.id) t ) rw
      from bgb b
      group by id ) z;

PostgreSQL – Use Trigger to Synchronize JSON Columns on Insert or Update

Actually, this is all you need:

NEW := jsonb_populate_record(NEW, NEW.json);

Per documentation:

jsonb_populate_record(base anyelement, from_json jsonb)
Expands the object in from_json to a row whose columns match the record type defined by base (see note below).

What's not documented: The row provided as first argument retains all values that are not overwritten (no matching key in the json value). I see no reason why this should change, but you cannot fully rely on it unless it's documented.

One thing to note - you wrote:

Setting the column to NULL if the corresponding json field does not exist is fine.

This retains all values with no matching key in the JSON value, which should be even better.

If "undocumented" is too uncertain for you, use the hstore operator #= doing exactly the same.

NEW := (NEW #= hstore(jsonb_populate_record(NEW, NEW.json)));

The hstore module should be installed in most systems anyway. Instructions:

Assign to NEW by key in a Postgres trigger

Both solution can be derived from my answer that Daniel already referenced:

How to set value of composite variable field using dynamic SQL

Function code

CREATE OR REPLACE FUNCTION json_fn()
  RETURNS TRIGGER AS
$func$
BEGIN
   NEW := jsonb_populate_record(NEW, NEW.json); -- or hstore alternative
   RETURN NEW;
END
$func$ LANGUAGE plpgsql;

Everything else in your setup looks right, just add a PK to testy:

CREATE TABLE testy (
    id   int   PRIMARY KEY REFERENCES testy_index
  , data jsonb NOT NULL
);

Tested in pg 9.4 and it works for me as advertised. I doubt that the PLv8 function can rival performance and simplicity.

Set other columns to NULL

As per comment:

CREATE OR REPLACE FUNCTION json_fn()
  RETURNS TRIGGER AS
$func$
DECLARE
  _j jsonb := NEW.json;  -- remember the json value
BEGIN
   NEW := jsonb_populate_record(NULL::testy, _j);
   NEW.json := _j;   -- reassign
   RETURN NEW;
END
$func$ LANGUAGE plpgsql;

Obviously, you need to make sure that the column name or your jsonb column does not appear as key name in the JSON value. And i wouldn't use json as column name, since it's a data type name and that can get confusing.