Convert JSONB to minified (no spaces) String - json

Question

If I convert a text value like {"a":"b"} to JSONB and then back to text a space () is added between the : and the ".
psql=> select '{"a":"b"}'::jsonb::text;
text
------------
{"a": "b"}
(1 row)
How can I convert a text to a jsonb, so I can use jsonb functions, and back to text to store it?

Answer 1

The JSON standard, RFC 8259, says "... Insignificant whitespace is allowed before or after any of the six structural characters". In other words, the cast from jsonb to text has no universal canonical form. The PostgreSQL cast convention (using spaces) is arbitrary.
So, we must to agree with the PostgreSQL's convention for CAST(var_jsonb AS text). When you need another cast convention, for example to debug or human-readable output, the built-in jsonb_pretty() function is a good choice.
Unfortunately PostgreSQL not offers other choices, like the compact one. So, you can overload jsonb_pretty() with a compact option:
CREATE or replace FUNCTION jsonb_pretty(
jsonb, -- input
compact boolean -- true for compact format
) RETURNS text AS $$
SELECT CASE
WHEN $2=true THEN json_strip_nulls($1::json)::text
ELSE jsonb_pretty($1)
END
$$ LANGUAGE SQL IMMUTABLE;
SELECT jsonb_pretty( jsonb_build_object('a',1, 'bla','bla bla'), true );
-- results {"a":1,"bla":"bla bla"}
See a complete discussion at this similar question.

Answer 2

From the docs:
https://www.postgresql.org/docs/12/datatype-json.html
"Because the json type stores an exact copy of the input text, it will preserve semantically-insignificant white space between tokens, as well as the order of keys within JSON objects. Also, if a JSON object within the value contains the same key more than once, all the key/value pairs are kept. (The processing functions consider the last value as the operative one.) By contrast, jsonb does not preserve white space, does not preserve the order of object keys, and does not keep duplicate object keys. If duplicate keys are specified in the input, only the last value is kept."
So:
create table json_test(fld_json json, fld_jsonb jsonb);
insert into json_test values('{"a":"b"}', '{"a":"b"}');
select * from json_test ;
fld_json | fld_jsonb
-----------+------------
{"a":"b"} | {"a": "b"}
(1 row)
If you want to maintain your white space or lack of it use json. Otherwise you will get a pretty print version on output with jsonb. You can json functions/operators on json type though not the jsonb operators/functions. More detail here:
https://www.postgresql.org/docs/12/functions-json.html
Modifying your example:
select '{"a":"b"}'::json::text;
text
-----------
{"a":"b"}

Answer 3

The way your question and comments are phrased, it really looks like you want replace().
We need to make the search as specific as possible to avoid messing with potentially embedded ': ' within the json payload, so it seems safer to match on the surrounding double quotes too, like:
replace('{"a":"b"}'::jsonb::text, '": "', '":"')