Indexes and EXPLAIN Plans

Indexes and EXPLAIN Plans

Now that we've covered schema-changing commands in What Is DDL? Data Definition Language Explained, data rules in What Are Constraints in SQL?, and practical command categories in Useful SQL Commands with Examples, we're ready for a question interviewers love:

"How would you make this query faster?"

A correct answer is rarely just: "Add an index." A stronger answer explains:

• what column or columns should be indexed,
• why the index helps the WHERE, JOIN, or ORDER BY,
• what tradeoff the index introduces,
• and how you would verify the change with an EXPLAIN plan.

Indexes belong in this Schema and Integrity section because they are database objects created alongside tables, constraints, and keys. They do not change the query result; they change how efficiently the database can find that result.