A SQL Heuristic: Ors Are Expensive
18 hours ago
- #SQL Optimization
- #Database Performance
- #Schema Design
- OR clauses in SQL queries can be significantly slower than AND clauses, with examples showing a 100x performance difference.
- Query planning relies on database statistics to decide the most efficient way to execute queries, especially when dealing with multiple filters.
- Indexes are more naturally suited for AND operations, making them less efficient for OR operations without careful planning.
- Schema design should consider access patterns, including whether searches will involve OR clauses, to optimize performance.
- Using secondary tables or restructuring data can improve query performance by avoiding expensive OR operations.
- The 'extension' pattern in schema design helps manage shared columns across tables, simplifying queries and improving efficiency.
- Practical tips include avoiding multiple columns with the same type and constraints and considering the use of secondary tables for one-to-many relationships.