MySQL Query Cache (Deprecated)

Also Known As

TL;DR

Explanation

MySQL's built-in query cache stored SELECT results and invalidated the entire table's cached results on any write. Under write-heavy workloads it became a contention point — every INSERT/UPDATE/DELETE invalidated potentially thousands of cached queries. It was deprecated in MySQL 5.7.20 and removed in 8.0. Modern alternatives are application-level caches (Redis, Memcached) which give control over cache keys, TTLs, and invalidation logic.

Common Misconception

Why It Matters

Common Mistakes

• Expecting query_cache_size to exist in MySQL 8 — it was fully removed.
• Relying on implicit database-level caching instead of explicit application caching.
• Using SELECT SQL_CACHE in queries targeting MySQL 8 — silently ignored.

Avoid When

• Do not set query_cache_size in MySQL 8 config — it causes startup errors as the option was fully removed.
• Do not rely on database-level caching — application-level caching is more predictable and scalable.

When To Use

• Implement application-level caching with Redis or Memcached — gives full control over cache keys and TTLs.

Code Examples

-- MySQL 8: query_cache_size no longer exists
-- query_cache_size = 64M -- This config option causes startup error in MySQL 8

// Application-level caching with Redis (preferred)
$cacheKey = 'user:' . $userId;
$user = $redis->get($cacheKey);
if ($user === null) {
    $stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?');
    $stmt->execute([$userId]);
    $user = $stmt->fetch();
    $redis->setex($cacheKey, 300, json_encode($user)); // 5 min TTL
}
return json_decode($user, true);

References

• ↗ https://dev.mysql.com/doc/refman/5.7/en/query-cache.html

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