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The following bug has been logged on the website:

Bug reference:      18000
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 16beta1
Operating system:   Ubuntu 22.04
Description:        

The following script:
BEGIN;
CREATE ACCESS METHOD heap2 TYPE TABLE HANDLER heap_tableam_handler;
CREATE MATERIALIZED VIEW heapmv USING heap AS SELECT 1::int;
ALTER MATERIALIZED VIEW heapmv SET ACCESS METHOD heap2;
DROP ACCESS METHOD heap2;
SELECT * FROM heapmv;
COMMIT;
SELECT * FROM heapmv;
DROP MATERIALIZED VIEW heapmv;

drops the access method heap2, but leaves the materialized view heapmv,
which then can not be accessed nor dropped:
ERROR:  cache lookup failed for access method 16390

You cannot dump the database, too:
pg_dump: error: query failed: ERROR:  cache lookup failed for access method
16390
pg_dump: detail: Query was: SELECT
pg_catalog.pg_get_viewdef('16391'::pg_catalog.oid) AS viewdef

But if the access method was set during a matview creation, it's deletion
is
prevented as expected:
CREATE ACCESS METHOD heap2 TYPE TABLE HANDLER heap_tableam_handler;
CREATE MATERIALIZED VIEW heapmv USING heap2 AS SELECT 1::int;
DROP ACCESS METHOD heap2;
ERROR:  cannot drop access method heap2 because other objects depend on it
DETAIL:  materialized view heapmv depends on access method heap2
HINT:  Use DROP ... CASCADE to drop the dependent objects too.

Reproduced on REL_15_STABLE .. master.