I've been doing a lot of tweaking of SQL queries as of late, trying to squeeze some performance out of a query by adding indexes, optimizing joins, and some other operations. To do a quick check to ensure that the queries return the same results, I've been hashing the results to see if I get an identical hash back.
SELECT md5(CAST((array_agg(f.* ORDER BY id)) AS TEXT)) as md5, encode( digest( CAST( (array_agg(f.* ORDER BY va.id)) AS TEXT ), 'sha1'), 'hex') as sha1 FROM foo f;
This solution was adapted from https://stackoverflow.com/a/13948327/206480.
sha1 requires pgcrypto to be installed.
This solution looks at individual rows. I have also covered how to hash an entire query result set.