Join Buffer (Block Nested Loop): Efficiently Joining Large Tables

A join buffer, also known as a block nested loop join, is a join algorithm used in database systems to efficiently perform a join operation between two tables. It's particularly useful when one or both of the tables being joined are too large to fit entirely in memory.

The join buffer algorithm works by dividing the larger table into blocks or chunks and reading these blocks into memory one at a time. For each block, it scans the smaller table and compares each row with the rows in the block. If a match is found, the join operation is performed and the result is stored in the join buffer.

Once the entire smaller table has been scanned, the join buffer contains all the matching rows from the join operation. The join buffer can then be used to return the result of the join operation to the user or to perform further operations, such as filtering or sorting.

Using a join buffer can significantly reduce the amount of disk I/O required for the join operation, as only a small portion of each table needs to be read into memory at a time. However, it does require multiple passes over the data and can be slower than other join algorithms, such as hash join or sort merge join, when the tables are small enough to fit entirely in memory.

Overall, the join buffer algorithm provides an efficient way to perform a join operation on large tables that cannot fit entirely in memory. It balances the use of memory and disk I/O to minimize the overall execution time of the join operation.