It is now possible to limit the query cache usage of individual users using settings profiles and settings constraints. More specifically, you can restrict the maximum amount of memory (in bytes) a user may allocate in the query cache and the the maximum number of stored query results. For that, first provide configurations query_cache_max_sizeand query_cache_max_entries in a user profile in users.xml, then make both settings readonly:

<profiles>
    <default>
        <!-- The maximum cache size in bytes for user/profile 'default' -->
        <query_cache_max_size>10000</query_cache_max_size>
        <!-- The maximum number of SELECT query results stored in the cache for user/profile 'default' -->
        <query_cache_max_entries></query_cache_max_entries>
        <!-- Make both settings read-only so the user cannot change them -->
        <constraints>
            <query_cache_max_size>
                <readonly/>
            </query_cache_max_size>
            <query_cache_max_entries>
                <readonly/>
            <query_cache_max_entries>
        </constraints>
    </default>
</profiles>

The main motivation is to prevent users from spamming the cache, i.e. evicting other user's query results. Other internal caches are generally also vulnerable to such misuse but the query cache makes misuse particularly easy.

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

It is now possible to define per-user quotas in the query cache