Hibernate version:2.1.8

It seems the standard query cache is always available even after session.close(), but not when selecting the entire object

for example, if we are selecting the table using

List l = createQuery("select a.col1,a.col2 from tableA a").list();

the cache collection will not be clean.

However

TableAData aa = createQuery(" from tableA");

The cache object will be gone from session.close

Am I missing something obvious?

Hibernate has a multi-level caching architecture.
1) Session cache
2) Secondary Cache - normally defined using ehcache.

The secondary cache data normally outlives any individual session.
There are two caching uses for the sescond level cache
1) Individual object data. (Note: Stored in the cache as a set of data rather than a full object).
2) A query result set. (Note: It stores sets of Individual data as in (1) Above and has a timestamp control mechanism.

In my view and experience you will find good benefits from selective use of second level cache for individual objects (using session.get or load etc) and less opporunities for benefits from the query cache.