SQL Set Operators Performance Impact

Understanding SQL Set Operators

Set operators in SQL include UNION, UNION ALL, INTERSECT, and EXCEPT. They combine results from two or more queries.

Each operator has different behaviors regarding duplicates and ordering, which influences performance.

• UNION combines results and removes duplicates.
• UNION ALL combines results without removing duplicates.
• INTERSECT returns rows common to both queries.
• EXCEPT returns rows from the first query not in the second.

Performance Characteristics of Set Operators

The main performance cost of set operators comes from duplicate elimination and sorting.

UNION requires removing duplicates, which often involves sorting or hashing the combined result set.

UNION ALL skips duplicate removal, making it faster and less resource-intensive.

• Duplicate elimination requires additional CPU and memory.
• Sorting large datasets can increase query execution time.
• INTERSECT and EXCEPT also perform duplicate elimination and filtering, which can be costly.

Optimizing Queries Using Set Operators

Choosing the right set operator and query design can improve performance.

When duplicates are not a concern, prefer UNION ALL over UNION.

Ensure appropriate indexes exist on columns involved in set operations to speed up filtering and sorting.

• Use UNION ALL if you do not need to remove duplicates.
• Avoid unnecessary use of INTERSECT and EXCEPT when simpler joins or filters suffice.
• Analyze query execution plans to identify bottlenecks caused by set operators.
• Consider breaking complex set operations into smaller parts if performance is poor.

Example: UNION vs UNION ALL Performance

Consider two tables with overlapping data. Using UNION removes duplicates but adds overhead.

Using UNION ALL returns all rows from both tables without duplicate checks, resulting in faster execution.

Example SQL Queries

Here are example queries demonstrating UNION and UNION ALL.