What should be considered when optimizing an SQL query?

When optimizing an SQL query, consider the following aspects:

• Use of indexes: ensure that appropriate indexes are created on columns involved in WHERE, JOIN, and ORDER BY clauses.
• Execution plan analysis (EXPLAIN): study how the DBMS executes the query to identify bottlenecks, such as full table scans.
• Minimize data retrieval: select only necessary columns, avoid SELECT *.
• Join optimization: use the correct type of join, avoid redundant joins.
• Early filtering: apply WHERE conditions as early as possible to reduce the amount of data processed.
• Use of aggregate functions and grouping wisely: avoid unnecessary grouping and sorting.
• Caching and materialized views: if queries are repeated, consider using cache or materialized views.

Example of using EXPLAIN in PostgreSQL:

EXPLAIN ANALYZE
SELECT * FROM orders WHERE customer_id = 123;

This will show how the DBMS plans to execute the query and where performance improvements can be made.