Starting from PostgreSQL 10, this feature is available. Transition tables for triggers are tables with old and new tuples, so your triggers can see what changed; they make the whole statement's changes available to you. Here is an example taken from the test suite at https://github.com/postgres/postgres/blob/a571c7f661a7b601aafcb12196d004cdb8b8cb23/src/test/regress/sql/plpgsql.sql:
CREATE TABLE transition_table_base (id int PRIMARY KEY, val text); CREATE OR REPLACE FUNCTION transition_table_base_upd_func() RETURNS trigger LANGUAGE plpgsql AS $$ DECLARE t text; l text; BEGIN t = ''; FOR l IN EXECUTE $q$ EXPLAIN (TIMING off, COSTS off, VERBOSE on) SELECT * FROM oldtable ot FULL JOIN newtable nt USING (id) $q$ LOOP ...
