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 R and S (the "outer" and "inner" join operands, respectively). Suppose | R | < | S | . In a traditional nested loop join, S will be scanned once for every tuple of R . If there are many qualifying R tuples, and particularly if there is no applicable index for the join key on S , this operation will be very expensive.
The block nested loop join algorithm improves on the simple nested loop join by only scanning S once for every group of R tuples. For example, one variant of the block nested loop join reads an entire page of R tuples into memory and loads them into a hash table. It then scans S , and probes the hash table to find S tuples that match any of the tuples in the current page of R . This reduces the number of scans of S that are necessary.
A more aggressive variant of this algorithm loads as many pages of R as can be fit in the available memory, loading all such tuples into a hash table, and then repeatedly scans S . This further reduces the number of scans of S that are necessary. In fact, this algorithm is essentially a special-case of the classic hash join algorithm.
The block nested loop runs in O ( P r P s / M ) I/Os where M is the number of available pages of internal memory and P r and P s is size of R and S respectively in pages. Note that block nested loop runs in O ( P r + P s ) I/Os if R fits in the available internal memory.
