Hive join Optimization-small table join large table

1. Small and large table join

When small tables and large tables are joinedSmall tables placed on the front sideHive caches small tables.

2. mapjoin

Use mapjoin to put small tables into the memory and match the large tables one by one on the map side to save the reduce workload.

Example:

Select/* + mapjoin (B) */a. A1, A. A2, B. B2 from tablea a join tableb B on A. A1 = B. B1

After version 0.7, you can also use the configuration to automatically optimize

Set hive. Auto. Convert. Join = true;

Hive join Optimization-small table join large table