Nested loop join

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A nested loop join is a naive algorithm that joins two sets by using two nested loops.[1] Join operations are important for database management.


Two relations and are joined as follows:

  For each tuple r in R do
     For each tuple s in S do
        If r and s satisfy the join condition
           Then output the tuple <r,s>

This algorithm will involve nr*bs+ br block transfers and nr+br seeks, where br and bs are number of blocks in relations R and S respectively, and nr is the number of tuples in relation R.

The algorithm runs in I/Os, where and is the number of tuples contained in and respectively and can easily be generalized to join any number of relations.

The block nested loop join algorithm is a generalization of the simple nested loops algorithm that takes advantage of additional memory to reduce the number of times that the relation is scanned.