Add support for "WITH UR" isolation clause on DB2
Project: Hibernate Core
Issue Type: Sub-task
Affects Versions: 3.3.1
Environment: HIbernate 3.3.1 / DB2 9.x
Reporter: Ricardo Fernandes
The question is basically the same as Steve's, i.e. to force DB2 to use as few locks
as possible on heavy load scenarios.
Although some might consider the use of the WITH UR clause a bad approach (since it allows
dirty reads), the fact is that there are some cases where this is in fact acceptable, such
as computing the total amount of rows of a query (for information purposes) or producing a
high-level listing of items with very little detailed info. The bottom line is: if I need
to trade performance for extremely accurate data, I surely want to be able to decide when
this should happen.
I've already performed the changes that makes it possible to use both the FOR READ
ONLY and the WITH UR clauses and I will be submitting a patch shortly so you can have a
look at it. The strategy I've used was basically the following:
1. Added two new methods on the Dialect class:
String getDatabaseReadOnlyString(String sql) - for adding the READ ONLY clause
String getDirtyReadsString(String sql) - for adding the WITH UR clause
2. Changed the Query interface in order to allow the user to say whether he/she wants the
query to allow dirty reads:
Query setAllowDirtyReads(boolean allowDirtyReads);
3. Added a default implementation on the AbstractQueryImpl which initializes the flag a
4. Added a similar attribute on the QueryParameters class
5. Changed the prepareQueryStatement() method of the Loader class (just after the useLimit
part in order:
a) Ask the dialect for the getDatabaseReadOnlyString() is there are no LockModes set (as
b) Ask the dialect for the getDirtyReadsString() is the queyParameters allows dirty
All the tests were well succeeded.
Hope you find these changes, at least, worth looking at.
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