Mastering PostgreSQL 12 - Third Edition

Hash joins

The second option is a hash join. The following strategy could be applied to solve our little problem. The following listing shows how a hash join works:

Hash join     Sequential scan table 1     Sequential scan table 2

Both sides can be hashed and the hash keys could be compared, leaving us with the result of the join. The problem here is that all of the values have to be hashed and stored somewhere.