HHH-13331 : Query Cache should not be allowed for queris having Non Cacheable Query Spaces

[debashishbharali]

HHH-13331 : Query Cache should not be allowed for queris having Non Cacheable Query Spaces

Analysis:

• When query is cacheable i.e. 'org.hibernate.cacheable=true', and is having query spaces which are non cacheable, then this may lead to the well known 'N+1' issue.
• I.e. the cached query result containing the 'N' ids, will further result in N queries of 'findById' to the database.

Expectation:

• Query Cache should not be allowed for queries having Non Cacheable Query Spaces. Either throw an exception while loading the query, or rather ignore the query cache in this case.
• With this, even high number of puts to 'UpdateTimestampCache' can be avoided. Which are originating from all the high number of transactional DMLs being executed to a query space which is non-cacheable.

Resolution : org.hibernate.loader.Loader.list() Mehod

• Maintained a set of all the cacheable 'query space (String)' - called it 'cacheableSpacesSet' etc
• Within the method 'Loader.list()', additional check of validating the current query's spaces from the initially maintained 'cacheableSpacesSet'
• With the above additional validation, even though the system is configured as 'hibernate.cache.use_query_cache=true', and corresponding query is set as cacheable (i.e. 'org.hibernate.cacheable=true'), still the 'cacheable boolean' within the list() method will be set as false for query having query spaces which are not cacheable.

Performance:

• This newly added additional validation uses 'HashSet.contains()' method internally. Which is having time complexity of O(1)
• On a simple laptop, for individual call with one query space, it took an additional 24us to 40us for the complete additional logic.
• On a server, this time is expected to reduce significantly.