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The following bug has been logged on the website:

Bug reference:      15913
Logged by:          Daniel Fiori
Email address:      zeroimpl@gmail.com
PostgreSQL version: 11.4
Operating system:   Debian 11.4-1.pgdg90+1
Description:        

I have some SQL which works fine in 9.x and 10.x, but fails in 11.x and 12.x
(I tested on various Docker images).

One of the functions declares a variable whose type matches a temporary
table. If I drop and recreate that temporary table twice in the same
session, I get an error like: "ERROR:  could not open relation with OID
xxx". This occurs on the second call to the DoSomething() function after the
temporary table has been recreated. 

---

BEGIN;

CREATE OR REPLACE FUNCTION BeginTest( arg TEXT ) RETURNS VOID AS $$
BEGIN
    CREATE TEMPORARY TABLE TestVal AS SELECT arg;
END
$$ LANGUAGE PLPGSQL;

SELECT BeginTest( NULL );

CREATE OR REPLACE FUNCTION EndTest() RETURNS VOID AS $$
BEGIN
    DROP TABLE TestVal;
END
$$ LANGUAGE PLPGSQL;

CREATE OR REPLACE FUNCTION DoSomething() RETURNS VOID AS $$
DECLARE
    varname TestVal;
BEGIN
    SELECT * INTO varname FROM TestVal;
END
$$ LANGUAGE PLPGSQL;

SELECT EndTest();

COMMIT;

---

Then in a different session run:
---

BEGIN;

SELECT BeginTest( 'abc' );
SELECT DoSomething();
SELECT EndTest();

SELECT BeginTest( 'def' );
SELECT DoSomething();
SELECT EndTest();

COMMIT;

---

Note if the above SQL is all run in the same session, a slightly different
error is reported: "ERROR:  type with OID xxx does not exist"

Based on the PG 11 release notes, it sounds like it's related to this
change:

> Allow PL/pgSQL to handle changes to composite types (e.g. record, row)
that happen between the first and later function executions in the same
session (Tom Lane). Previously, such circumstances generated errors.