Block nested loop

A block-nested loop (BNL) is an algorithm used to join two relations in a relational database.

This algorithm is a variation on the simple nested loop join used to join two relations ${\displaystyle R}$ and ${\displaystyle S}$ (the "outer" and "inner" join operands, respectively). Suppose ${\displaystyle |R|<|S|}$. In a traditional nested loop join, ${\displaystyle S}$ will be scanned once for every tuple of ${\displaystyle R}$. If there are many qualifying ${\displaystyle R}$ tuples, and particularly if there is no applicable index for the join key on ${\displaystyle S}$, this operation will be very expensive.

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