Hibernate internally uses a cache that maps HQL statements (as strings) to query plans. The cache consists of a bounded map limited by default to 2048 elements (configurable). All HQL queries are loaded through this cache. In case of a miss, the entry is automatically added to the cache. This makes it very susceptible to thrashing - a scenario in which we constantly put new entries into the cache without ever reusing them and thus preventing the cache from bringing any performance gains (it even adds some cache management overhead). To make things worse, it is hard to detect this situation by chance - you have to explicitly profile the cache in order to notice that you have a problem there. I will say a few words on how this could be done later on.
So the cache thrashing results from new queries being generated at high rates. This can be caused by a multitude of issues. The two most common that I have seen are - bugs in hibernate which cause parameters to be rendered in the JPQL statement instead of being passed as parameters and the use of an "in" - clause.
Due to some obscure bugs in hibernate, there are situations when parameters are not handled correctly and are rendered into the JPQL query (as an example check out HHH-6280). If you have a query that is affected by such defects and it is executed at high rates, it will thrash your query plan cache because each JPQL query generated is almost unique (containing IDs of your entities for example).
The second issue lays in the way that hibernate processes queries with an "in" clause (e.g. give me all person entities whose company id field is one of 1, 2, 10, 18). For each distinct number of parameters in the "in"-clause, hibernate will produce a different query - e.g. select x from Person x where x.company.id in (:id0_) for 1 parameter, select x from Person x where x.company.id in (:id0_, :id1_) for 2 parameters and so on. All these queries are considered different, as far as the query plan cache is concerned, resulting again in cache thrashing. You could probably work around this issue by writing a utility class to produce only certain number of parameters - e.g. 1, 10, 100, 200, 500, 1000. If you, for example, pass 22 parameters, it will return a list of 100 elements with the 22 parameters included in it and the remaining 78 parameters set to an impossible value (e.g. -1 for IDs used for foreign keys). I agree that this is an ugly hack but could get the job done. As a result you will only have at most 6 unique queries in your cache and thus reduce thrashing.
So how do you find out that you have the issue? You could write some additional code and expose metrics with the number of entries in the cache e.g. over JMX, tune logging and analyze the logs, etc. If you do not want to (or can not) modify the application, you could just dump the heap and run this OQL query against it (e.g. using mat):
SELECT l.query.toString() FROM INSTANCEOF org.hibernate.engine.query.spi.QueryPlanCache$HQLQueryPlanKey l It will output all queries currently located in any query plan cache on your heap. It should be pretty easy to spot whether you are affected by any of the aforementioned problems.
As far as the performance impact goes, it is hard to say as it depends on too many factors. I have seen a very trivial query causing 10-20 ms of overhead spent in creating a new HQL query plan. In general, if there is a cache somewhere, there must be a good reason for that - a miss is probably expensive so your should try to avoid misses as much as possible. Last but not least, your database will have to handle large amounts of unique SQL statements too - causing it to parse them and maybe create different execution plans for every one of them.