Hibernate-SQL很快，但是查询仍然很慢

Question

I'm running Hibernate 4.1 with Javassist runtime instrumentation running on top of Hikari pool, on top of Oracle 12c. JDK is 1.7.

I have a query that runs pretty fast on the database and fetches about 500 entities in Hibernate. The query runtime, according to JProfiler is quite small, about 11 ms, but in total Query.list runs about 7 seconds.

I've tried removing all filters and it shows that most of the time is spent in Javaassist and other Hibernate-related reflection calls (like AbstractProxyHandler and such). I read that the reflection overhead should be pretty small, but it seems like it is not, and it seems like it is too much.

Could you please advise what could be the bottleneck here?

Answer 1

Make sure the object you are retrieving does not have sub-objects that are being fetched lazily as a SELECT instead of eagerly as a JOIN. This can result in a behavior known as SELECT N + 1, where Hibernate ends up running a query to get the 500 objects from their respective table, then an additional query for each object to get the child object. If you have 4 or 5 relationships that are being fetched as SELECT statements for each record, and you have 500 records, suddenly you're running around 2000 queries in order to get the List.

I would recommend turning on the SQL logging for Hibernate to see which queries it's running. If it dumps out a really long list of SELECT queries when you're fetching your list, look at your mapping file to see how your relationships are set up. Try adjusting them to be a fetch="join" and see if those queries go away and if the performance improves.

Here are some possibly related Stack Overflow questions that may be able to provide more specific details.

Hibernate FetchMode SELECT vs JOIN

What is N+1 SELECT query issue?

Something else to note about profilers and other tools of that nature. Often when tracking down a performance issue, a particular block of code will show up as where the most time is being spent. The common conclusion people tend to jump to is that the block of code in question is slow. In your case, you seem to be observing Hibernate's reflective code as being where the time is spent. It is very important to consider that this code itself might not actually be slow, but it is where the time is being spent because it is being called frequently due to algorithm complexity . I have found in many problems, serialization or reflection appears to be slow, when the reality is that the code is used to communicate with an external resource, and that resource is being called 1000s of times when it should only be called a handful. Making 1000s of queries to fetch your list will result in your sampling showing that a lot of time is being spent in the code that processes those queries. Be careful not to mistake code that is called often due to a design/configuration issue for code that is slow. The problem very likely does not lay in hibernate's use of reflection, since reflection generally isn't slow on the order of seconds.