[PATCH] PREPARE TRANSACTION unexpected behavior with TEMP TABLE - Mailing list pgsql-hackers

set max_prepared_transactions to 1 (or any non zero value), this is to enable the “prepare transaction”.

postgres=# create temp table fullname (first text, last text);
CREATE TABLE
postgres=# BEGIN;
BEGIN
postgres=*# create function longname(fullname) returns text language sql
postgres-*# as $$select $1.first || ' ' || $1.last$$;
CREATE FUNCTION
postgres=*# prepare transaction 'mytran';
ERROR:  cannot PREPARE a transaction that has operated on temporary objects

The problem is if we again try to create the same function in the “PREPARE TRANSACTION” as below.
postgres=# BEGIN;
BEGIN
postgres=*# create function longname(fullname) returns text language sql
as $$select $1.first || ' ' || $1.last$$;
CREATE FUNCTION
postgres=*# PREPARE transaction 'mytran';
PREPARE TRANSACTION

Now, this time we succeed and not getting the above error (”ERROR:  cannot PREPARE a transaction that has operated on temporary objects), like the way we were getting with BLOCK-1

This is happening because we set MyXactFlags in relation_open function call, and here relation_open is getting called from load_typcache_tupdesc, but in the second run of “create function…” in the above #2 block will not call load_typcache_tupdesc because of the below condition(typentry->tupDesc == NULL) in  lookup_type_cache().

        /*
         * If it's a composite type (row type), get tupdesc if requested
         */
        if ((flags & TYPECACHE_TUPDESC) &&
                typentry->tupDesc == NULL &&
                typentry->typtype == TYPTYPE_COMPOSITE)
        {
                load_typcache_tupdesc(typentry);
        }

We set typentry->tupDesc to non NULL(and populates it with proper tuple descriptor in the cache) value during our first call to “create function…” in BLOCK-1.
We have logic in file xact.c::PrepareTransaction() to simply error out if we have accessed any temporary object in the current transaction but because of the above-described issue of not setting XACT_FLAGS_ACCESSEDTEMPNAMESPACE in MyXactFlags second run of “create function..” Works and PREPARE TRANSACTION succeeds(but it should fail).