Re: jsonb - jsonb operators

Dmitry Dolgov <9erthalion6(at)gmail(dot)com> writes:
>> if there's any future intention to add a delete operator that removes
> element/pair matches?

> I think the operator (jsonb - jsonb) is logical because we have a shallow
> concatenation function (something like a "union" operation), but we have
> nothing like "set difference" and "intersection" functions. Actually, I
> thought to implement these functions (at least for jsonbx). But of course
> this function should be quite simple and consider only full key/value
> matching as a target.

I am wary of this proposal because it seems to be taking little
account of the fact that there *already is* a jsonb minus operator,
two of them in fact. For example

regression=# select '["a","b","c"]'::jsonb - 'b';
?column?
------------
["a", "c"]
(1 row)

regression=# select '{"a":1, "b":2}'::jsonb - 'b';
?column?
----------
{"a": 1}
(1 row)

The proposed full-match semantics don't seem to me to be consistent with
the way that the existing operator works.

Another rather nasty problem is that the latter case works at all,
ie the parser will decide the unknown literal is "text" so that it can
apply "jsonb - text", there being no other plausible choice. If there
were a "jsonb - jsonb" operator, the parser would prefer that one, due
to its heuristic about assuming that an unknown literal is of the same
type as the other operator input. So adding such an operator will almost
certainly break queries that work in 9.5. Maybe it's worth adding one
anyway, but I don't think the case for its usefulness has been proven
to the point where we should create compatibility issues to get it.

regards, tom lane