Rolling-hash aggregator for PostgreSQL, allowing fast exact comparison of columns across tables/databases

hash_agg.sql

CREATE FUNCTION hash_agg_tfunc(bytea, anyelement) RETURNS bytea AS $$
  SELECT sha256(convert_to($2::text, 'UTF8') || $1)
$$ LANGUAGE SQL;

CREATE AGGREGATE hash_agg (anyelement) (
  sfunc = hash_agg_tfunc,
  stype = bytea,
  initcond = '{}'
);