Hibernate produce different SQL for every query

Question

I've just tested my application under the profiler and found out that sql strings use about 30% of my memory! This is bizarre.

There are a lot of strings like this stored in app memory. This is SQL queries generated by hibernate, note the different numbers and trailing underscores:

select avatardata0_.Id as Id4305_0_,...... where avatardata0_.Id=? for update select avatardata0_.Id as Id4347_0_,...... where avatardata0_.Id=? for update 

Here is the part I can't understand. Why does hibernate have to generate different sql strings with different identifiers like "Id4305_0_" for each query? Why can't it use one query string for all identical queries? Is this some kind of trick to bypass query caching?

I would greatly appreciate if someone would describe me why it happening and how to avoid such resource wasting.

UPDATE

Ok. I found it. I was wrong assuming memory leak, It was my fault. Hibernate is working as intended.

My app created 121(!) SessionFactories in 10 threads, they produced about 2300 instances of SingleTableEntityPersisters. And each SingleTableEntityPersister generates about 15 SQL queries with different identifiers. Hibernate was forced to generate about 345.000 different SQL queries. Everything is fine, nothing weird :)

Answer 1

There is a logic behind the query string that hibernate generates. Its primary aim is to get unique aliases for tables and columns names.

From your query,

select avatardata0_.Id as Id4305_0_,...... where avatardata0_.Id=?

avatardata0_ ==> avatardata is the alias of the table and 0_ is appended to indicate it is the first table in the query. So if it were the second table(or Entity) in the query it should have been shown as avatardata1_. It uses the same logic for the column aliases.

So, this way all the possible conflicts are avoided.

You are seeing theses queries because you have turns on the show_sql flag the configuration. This is intended for the debugging of queries. Once you application started working you are supposed turn it off.

Read more on the API docs here.

I am not much aware of the memory consumption part, but you repeat your tests with the above flag turned off and see if there is any improvement.