Databases & Data Storage
Query Optimization
Definition
Query optimization is the process of choosing the most efficient way to execute a query in a database. This is handled by a component of the DBMS called the query optimizer or query planner.
Why It Matters
The performance difference between a good and a bad query plan can be orders of magnitude. Good query optimization is critical for the performance of any database-driven application.
Contextual Example
To join two tables, the optimizer might choose between a "nested loop join," a "hash join," or a "merge join" based on the size of the tables, the available indexes, and the data distribution. Its goal is to find the plan with the lowest estimated cost.
Common Misunderstandings
- While the optimizer is automatic, database administrators can influence it by creating appropriate indexes, keeping statistics up-to-date, and sometimes rewriting queries.
- The `EXPLAIN` command in SQL allows you to see the execution plan chosen by the optimizer for a given query.