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• Nested-Loop Join Algorithm

A simple nested-loop join (NLJ) algorithm reads rows from the first table in a loop one at a time, passing each row to a nested loop that processes the next table in the join. This process is repeated as many times as there remain tables to be joined.

Assume that a join between three tablest1,t2, andt3 is to be executed using the following join types:

Table   Join Typet1      ranget2      reft3      ALL

If a simple NLJ algorithm is used, the join is processed like this:

for each row in t1 matching range {  for each row in t2 matching reference key {    for each row in t3 {      if row satisfies join conditions, send to client    }  }}

Because the NLJ algorithm passes rows one at a time from outer loops to inner loops, it typically reads tables processed in the inner loops many times.