What is query caching?

Query caching is a technique used in databases to store the results of a query so that if the same query is executed again, the database can return the stored result instead of executing the query all over again. This can significantly improve performance, especially for complex or frequently run queries.

Here’s a detailed breakdown:

1. How Query Caching Works

• When a query is executed for the first time, the database processes it normally and stores the result set in a special memory area called the query cache.
• If the same query (same SQL text and same parameters) is executed again, the database checks the cache first:
  • If a cached result exists and the underlying data has not changed, the cached result is returned immediately.
  • If the data has changed, the cache is invalidated, and the query is executed again.

2. Benefits of Query Caching

• Faster response times: Avoids repeated computation for the same query.
• Reduced server load: Less CPU and I/O usage because the database doesn’t execute the query again.
• Efficiency for reporting: Especially useful for read-heavy applications or reporting dashboards.

3. Limitations

• Cache invalidation: If the underlying table data changes (INSERT, UPDATE, DELETE), cached queries using that table are invalidated.
• Memory usage: Cached results occupy memory, which can be a problem if too many queries are cached.
• Query-specific: Only identical queries (including whitespace, letter case, etc.) benefit from caching.

4. Example in MySQL

Suppose you have a table employees:

SELECT * FROM employees WHERE department_id = 5;

• First execution: MySQL runs the query and stores the result in the query cache.
• Second execution: If no changes have been made to the employees table, MySQL retrieves the result directly from the cache, skipping actual query execution.

5. Important Notes

• In MySQL 8.0, the query cache is removed, because it caused more overhead in modern workloads. Applications now rely more on application-level caching (like Redis or Memcached).
• Query caching is most beneficial for read-heavy databases where the data doesn’t change frequently.