Hash join是数据库实现join操作的一种算法。

  • First prepare a hash table of the smaller relation. The hash table entries consist of the join attribute and its row.
  • Once the hash table is built, scan the larger relation and find the relevant rows from the smaller relation by looking in the hash table.

有3种基本的join实现算法：

Three fundamental algorithms for performing a join operation exist: nested loop join, sort-merge join and hash join.

Nested loop join就是两个简单粗暴的for loop。