# Merge Join 
+ Merge Join example
+ a is the primary key and is indexed at (b, c)


![left](./student-guide/diagram/17-Table-join-class-operator-01-01.png)

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Merge joins take up relatively little memory compared to hash joins, but are slower than hash joins. When there is a large amount of data, or when there is not enough memory, we can try to use merge join. In a merge join, the data of a join group from the build side is read into memory, and then the data of the probe side is read, and each row of data from the probe side is compared with a complete join group on the build side.