Hash Joins

The primary alternative to a Merge Join, a Hash Join doesn't sort its input. Instead, it creates a hash table from each row of the inner table, scanning for matching ones in the outer. The output will not necessarily be in any useful order.

A query to find all the products that have at one point been ordered by any customer shows a regular Hash Join:

EXPLAIN ANALYZE SELECT prod_id,title FROM products p WHERE EXISTS (SELECT 1 FROM orderlines ol WHERE ol.prod_id=p.prod_id);
QUERY PLAN 
-------------------
 Hash Join  (cost=1328.16..2270.16 rows=9724 width=19) (actual time=249.783..293.588 rows=9973 loops=1)
   Hash Cond: (p.prod_id = ol.prod_id)
   ->  Seq Scan on products p  (cost=0.00..201.00 rows=10000 width=19) (actual time=0.007..12.781 rows=10000 loops=1)
   ->  Hash  (cost=1206.62..1206.62 rows=9724 width=4) (actual time=249.739..249.739 rows=9973 loops=1)
         Buckets: 1024  Batches: 1  Memory Usage: 234kB
         ->  HashAggregate  (cost=1109.38...