check from clause postgresql code example

Example: how to see constraints in postgresql

SELECT con.*
       FROM pg_catalog.pg_constraint con
            INNER JOIN pg_catalog.pg_class rel
                       ON rel.oid = con.conrelid
            INNER JOIN pg_catalog.pg_namespace nsp
                       ON nsp.oid = connamespace
       WHERE nsp.nspname = '<schema name>'
             AND rel.relname = '<table name>';