QUERY PLAN
Aggregate (cost=27.64..27.65 rows=1 width=8)
-> Hash Right Join (cost=10.45..24.09 rows=20 width=100)
Hash Cond: (cl.id = c.client_class)
-> Seq Scan on tbl_client_classes cl (cost=0.00..12.50 rows=250 width=44)
-> Hash (cost=10.20..10.20 rows=20 width=60)
-> Seq Scan on tbl_clients c (cost=0.00..10.20 rows=20 width=60)
Filter: (client_class IS NOT NULL)