Define nested loop join algorithm and describe how can execute query
The nested loop join algorithm is a join method used in relational databases to combine data from two or more tables based on a common attribute. It is called "nested loop" because it iterates through each row of one table (outer loop) and for each row, iterates through each row of the other table (inner loop) to find matching values.
Here is a step-by-step description of how the nested loop join algorithm can execute a query:
1. Parse the query:
- Identify the tables involved in the join.
- Determine the join condition or attribute that will be used to match rows between tables.
- Identify other conditions or filters applied to the query.
2. Start with the outer loop:
- Retrieve the first row from the outer table.
- For each row in the outer table, proceed to the inner loop.
3. Inner loop:
- Retrieve the first row from the inner table.
- For each row in the inner table, check if the join condition is satisfied (i.e., the common attribute values match).
- If the join condition is satisfied, combine the selected attributes from both rows into a result row. Add this result row to the output.
4. Repeat steps 3 until all rows in the inner table have been checked.
5. Move to the next row in the outer table and repeat steps 3 and 4 until all rows in the outer table have been checked.
6. Apply any additional conditions or filters specified in the query. For each result row, check if it satisfies these conditions. If not, remove the row from the output.
7. Return the final set of result rows.
It's worth noting that the nested loop join algorithm can be efficient when one or both tables are small. However, it can become costly in terms of performance when dealing with large tables, as it requires scanning the entire inner table for each row of the outer table. In such cases, alternative join algorithms like merge join or hash join may be more suitable.