Fix type-checking of RECORD-returning functions in FROM, redux.

Commit 2ed8f9a01 intended to institute a policy that if a
RangeTblFunction has a coldeflist, then the function return type is
certainly RECORD, and we should use the coldeflist as the source of
truth about what the columns of the record type are.  When the
original function has been folded to a constant, inspection of the
constant might give a different answer.  This situation will lead to
a tuple-type-mismatch error at execution, but up until that point we
need to consistently believe the coldeflist, or we'll have problems
from different bits of code reaching different conclusions.

expandRTE didn't get that memo though, and would try to produce a
tupdesc based on the constant in this situation, leading to an
assertion failure.  (Desultory testing suggests that non-assert
builds often manage to give the expected error, although I also
saw a "cache lookup failed for type 0" error, and it seems at
least possible that a crash could happen.)

Some other callers of get_expr_result_type and get_expr_result_tupdesc
were also being incautious about this.  While none of them seem to
have actual bugs, they're working harder than necessary in this case,
besides which it seems safest to have an explicit policy of not using
those functions on an RTE with a coldeflist.  Adjust the code
accordingly, and add commentary to funcapi.c about this policy.

Also fix an obsolete comment that claimed "get_expr_result_type()
doesn't know how to extract type info from a RECORD constant".
That hasn't been true since commit d57534740.

Per bug #18422 from Alexander Lakhin.
As with the previous commit, back-patch to all supported branches.

Discussion: https://postgr.es/m/18422-89ca86c8eac5246d@postgresql.org

2 files changed, 4 insertions, 0 deletions

diff --git a/src/test/regress/expected/rangefuncs.out b/src/test/regress/expected/rangefuncs.out
index 2bda957f43e..397a8b35d6d 100644
--- a/src/test/regress/expected/rangefuncs.out
+++ b/src/test/regress/expected/rangefuncs.out
@@ -2498,3 +2498,6 @@ with a(b) as (values (row(1,2,3)))
 select * from a, coalesce(b) as c(d int, e int, f float);  -- fail
 ERROR:  function return row and query-specified return row do not match
 DETAIL:  Returned type integer at ordinal position 3, but query expects double precision.
+select * from int8_tbl, coalesce(row(1)) as (a int, b int);  -- fail
+ERROR:  function return row and query-specified return row do not match
+DETAIL:  Returned row contains 1 attribute, but query expects 2.
diff --git a/src/test/regress/sql/rangefuncs.sql b/src/test/regress/sql/rangefuncs.sql
index 06d598c5e9b..3c47c98e113 100644
--- a/src/test/regress/sql/rangefuncs.sql
+++ b/src/test/regress/sql/rangefuncs.sql
@@ -824,3 +824,4 @@ with a(b) as (values (row(1,2,3)))
 select * from a, coalesce(b) as c(d int, e int, f int, g int);  -- fail
 with a(b) as (values (row(1,2,3)))
 select * from a, coalesce(b) as c(d int, e int, f float);  -- fail
+select * from int8_tbl, coalesce(row(1)) as (a int, b int);  -- fail