(Postgres) Function to preview locks acquired by a schema change

show_locks.sql

-- Usage:
-- ---------------------------
-- BEGIN;
--   <statements to analyze: ALTER TABLE ...whatever...;>
-- SELECT * FROM show_locks();
-- ROLLBACK;
-- ---------------------------
CREATE OR REPLACE FUNCTION show_locks()
	RETURNS TABLE (
		"lock_type" VARCHAR,
		"table" NAME
	)
AS $$
	-- DO NOT USE ON PRODUCTION DB!
	-- (this will actually acquire the locks)
	-- use a local copy of the db, preferably empty
	SELECT
		pl.mode,
		pc.relname
	FROM pg_locks pl
	    LEFT JOIN pg_class pc
	    	ON pc.oid = pl.relation
	WHERE
	    virtualtransaction=(
	    	SELECT virtualtransaction FROM pg_locks
	    		WHERE
	    			transactionid::text = (txid_current() % (2^32)::bigint)::text
	    			-- compare int to xid, see https://dba.stackexchange.com/a/123183/10371 
	    			AND locktype='transactionid'
	    	LIMIT 1
	    ) AND
		pc.relnamespace >= 2200
		-- 2200 is a magic number to exclude pg_* internal locks
		-- see https://stackoverflow.com/a/37110529/202168
	ORDER BY (
	    -- hack to order results by mode, in the order below
	    -- (i.e. 'strongest' lock to weakest)
	    -- https://www.postgresql.org/docs/9.3/static/explicit-locking.html#LOCKING-TABLES
	    -- see https://stackoverflow.com/a/4088794/202168
		pl."mode" != 'AccessExclusiveLock',
		pl."mode" != 'ExclusiveLock',
		pl."mode" != 'ShareRowExclusiveLock',
		pl."mode" != 'ShareLock',
		pl."mode" != 'ShareUpdateExclusiveLock',
		pl."mode" != 'RowExclusiveLock',
		pl."mode" != 'RowShareLock',
		pl."mode" != 'AccessShareLock',
	    --	
		relname
	);
$$ LANGUAGE sql;