Executing a JOIN operation within a database involves several steps, including:
- Query Parsing and Analysis: The database engine parses the SQL query and analyzes the JOIN conditions and table relationships. It identifies the tables to be joined and the join predicates that specify the matching criteria.
- Join Algorithm Selection: The database engine selects an appropriate join algorithm based on the query structure, table sizes, and available join indexes. Common join algorithms include Nested Loops Join, Hash Join, and Merge Join.
- Join Order Determination: The database engine determines the optimal join order using join reordering techniques. It evaluates different join orders based on estimated costs and considers factors like table sizes, join selectivity, and available indexes.
- Table Access and Data Retrieval: The database engine accesses the relevant tables and retrieves the required data. It may utilize indexes to efficiently locate matching rows and minimize data access overhead.
- Join Predicate Evaluation: The database engine evaluates the join predicates for each pair of rows from the joined tables. It compares the specified join columns and determines whether the rows satisfy the join conditions.
- Join Result Construction: The database engine constructs the join result by combining the matching rows from the joined tables. It forms the output table based on the JOIN clause’s specified columns and join type.
- Join Result Sorting and Aggregation: If necessary, the database engine sorts the join result based on specified ORDER BY clauses or performs aggregations using GROUP BY and HAVING clauses.
- Join Result Materialization: The database engine materializes the join result, either by storing it temporarily in memory or writing it to an intermediate file.
- Join Result Processing: The database engine processes the materialized join result, applying any additional filtering, sorting, or aggregation operations specified in the query.
- Join Result Presentation: The database engine presents the final join result to the user or application, typically as a table or an iterator over the rows.