Re: domains, case statements, functions: bug?

Adrian Klaver <adrian.klaver@gmail.com> writes:
> test=> create function f(t) returns m as $$ select case when true then
> $1.c end $$ language sql;
> ERROR:  return type mismatch in function declared to return m
> DETAIL:  Actual return type is numeric.

pg_typeof is somewhat helpful here:

regression=# select pg_typeof(t.c) from t;
 pg_typeof
-----------
 m
(1 row)

regression=# select pg_typeof(case when true then t.c end) from t;
 pg_typeof
-----------
 numeric
(1 row)

The reason for this is that CASE uses select_common_type() to infer the
output type, and select_common_type intentionally discriminates against
domain types.  The comment therein says:

     * If all input types are valid and exactly the same, just pick that type.
     * This is the only way that we will resolve the result as being a domain
     * type; otherwise domains are smashed to their base types for comparison.

So the way to get a CASE to return a domain type is to be sure you
provide an ELSE with the same result type:

regression=# select pg_typeof(case when true then t.c else null::m end) from t;
 pg_typeof
-----------
 m
(1 row)

            regards, tom lane