"could not open relation with OID XXX" when using recreated index in sql function

Hi guys,

I think I found a bug in sql (not plpgsql) functions.
This is the error I’m getting:

ERROR: XX000: could not open relation with OID 13053550
CONTEXT: SQL function "fail" statement 3
LOCATION: relation_open, heapam.c:1130

It happens when I try to recreate an index and use it afterwards (both within the function).

My postgres version:
PostgreSQL 9.6.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.2 20140120 (Red Hat 4.8.2-16), 64-bit

Here is some sql to reproduce it:

BEGIN;

CREATE TABLE test_1 (
test_id BIGINT PRIMARY KEY,
other_id BIGINT NOT NULL
);
CREATE INDEX test_1_other_idx ON test_1(other_id);

— Make sure query planner uses the index scan
SET enable_seqscan=off;

CREATE OR REPLACE FUNCTION fail() RETURNS bigint
LANGUAGE sql AS
$$
-- Re-create the index
DROP INDEX IF EXISTS test_1_other_idx;
CREATE INDEX test_1_other_idx ON test_1(other_id);

-- Fail!
SELECT test_id FROM test_1 WHERE other_id = 1000;
$$;

SELECT fail();

ROLLBACK;

If you modify this function to use plpgsql, it won’t fail anymore:

CREATE OR REPLACE FUNCTION ok() RETURNS bigint
LANGUAGE plpgsql AS
$$
BEGIN

-- Re-create the index
DROP INDEX IF EXISTS test_1_other_idx;
CREATE INDEX test_1_other_idx ON test_1(other_id);

RETURN (SELECT test_id FROM test_1 WHERE other_id = 1);

END
$$;

SELECT ok();

It also works fine when you paste contents of fail() function into psql session.

Best Regards,
Krystian Szladewski