Seems that parser not using precedence ideal order, and that casting obligation losts performance.
The first problem is self-evident in this example: SELECT '{"x":1}'::jsonb || (('{"A":{"y":2}}'::jsonb)->'A') -- it is ok, expected result with (x,y) SELECT '{"x":1}'::jsonb || '{"A":{"y":2}}'::jsonb)->'A' -- non-expected result (y). Higher precedence <https://en.wikipedia.org/wiki/Order_of_operations> most be for -> operator, that is like an object-oriented *path* operator, always higher than algebric ones. Other problem is using this operation as SQL function, CREATE FUNCTION term_lib.junpack(jsonb,text) RETURNS JSONB AS $f$ SELECT ($1-$2)::JSONB || ($1->>$2)::JSONB; $f$ LANGUAGE SQL IMMUTABLE; without casting produce error. Perhaps will be "more friendly" without cast obligation, and it is a performance problem, the abusive use of castings losts performance.