What is a slow query in the context of working with databases?

A slow query is an SQL query that takes significantly longer to execute than expected or acceptable for the given system, negatively impacting application performance and user experience.

Reasons for slow queries:

• Lack of or suboptimal indexes.
• Poorly designed database structure (lack of normalization, redundancy).
• Careless use of SELECT *.
• Redundant or inefficient JOIN operations.
• Lack of result limiting (LIMIT).
• Large data volumes.
• Table or row locks.
• Incorrect database or server configuration.

Tools for identifying slow queries:

• Slow query logs in DBMS.
• Performance monitoring tools (e.g., New Relic, Datadog).
• Query profilers (EXPLAIN in MySQL, EXPLAIN PLAN in Oracle).

Example of query analysis using EXPLAIN:

-- Analyzing a query in MySQL
EXPLAIN SELECT customers.name, orders.order_date
FROM customers
JOIN orders ON customers.id = orders.customer_id
WHERE orders.order_date > '2023-01-01';

Analyzing the EXPLAIN output helps understand how the database executes the query: join order, used indexes, number of scanned rows, etc.

Consequences of slow queries:

• Increased application response time.
• Increased load on the database server.
• Locking of other queries.
• Deterioration of user experience.
• Increased infrastructure costs.