Define sort-merge join and explain how can execute query
Sort-merge join is a join algorithm used in relational database management systems (RDBMS) to combine two or more tables based on a specified condition. It involves sorting the tables being joined based on the join column(s), then merging the sorted tables to find matching rows.
The execution of a query using sort-merge join typically follows these steps:
1. Sorting: The tables being joined are sorted based on the join column(s). This can be done using an external sorting algorithm, which utilizes disk storage for handling large amounts of data, or an in-memory sorting algorithm for smaller datasets that fit into memory.
2. Merging: The sorted tables are sequentially scanned and merged based on the join condition. The merge process compares the join column values of the current row from one table with the join column values of the current row from the other table. If they match, a new row containing the combined data is added to the result set.
3. Output: The matched rows generated during the merge process are collected to form the final result set of the query. Additional operations like filtering and projection may be applied to the result set based on the query requirements.
It's worth noting that sort-merge join requires both tables to be sorted, which can be a resource-intensive operation. Therefore, it is typically used when joining large tables or when the join condition is complex. The algorithm's performance can be improved by using indexing on the join columns to reduce the sorting time and improve overall query execution time.