Yeah, the idea that this won't cause possibly significant pain is quite wrong. Quite by accident I came across an example just this morning where rewriting as a CTE makes a big improvement.
I wrote this query:
select (json_populate_record(null::mytype, myjson)).* from mytable;
It turned out that this was an order of magnitude faster:
with r as ( select json_populate_record(null::mytype, myjson) as x from mytable ) select (x).* from r;
Except I suspect we at least have a chance to detect the above and not de-optimize it by evaluating "json_populate_record" once for every column in mytype.
The now idiomatic solution to the above is to use LATERAL so the above CTE is no longer actually a required workaround.
