PreparedStatement in two servlets

Question

Hello,
I've got a doubt concerning PreparedStatement with JDBC.
If I have the same statement in two different servlets, is my SQL compiled just once in the DBMS ?
So, if I create a new Instance of PreparedStatement that was previously created in another servlet the statement is not created again.
Is it true?
Thanks.
C.

Answer 1

The performance impact of PreparedStatement varies widely from DBMS to DBMS (and driver implementation to driver implementation).
At one end, some systems merely implement PreparedStatement on top of Statement; the SQL is treated identically in the database no matter which statement type you use.
At the other end are DBMS's such as Oracle, that work really hard to optimize PreparedStatement and have several internal layers.
In Oracle, the compilation step is called a "parse", and there are 3 possibilities when executing a SQL statement using a PreparedStatement:
no parse
soft parse
hard parse
A hard or soft parse will occur when the execution connection.PrepareStatement is called. The ideal situation for each SQL statement is "parse once execute many".
In a hard parse, Oracle looks at the SQL, computes a hashcode from the SQL text and looks for an appropriate execution plan in its caches. A cache miss occurs, Oracle prepares the execution plan, saves it in cache keyed by the hashcode, and binds the execution plan to the PreparedStatement (effectively speaking, at least, I'm not sure how the implementation achieves that). It's then ready for parameter binding and execute().
In a soft parse, Oracle looks at the SQL, computes a hashcode from the SQL text and looks for an appropriate execution plan in its caches. A cache hit occurs, Oracle finds an existing execution plan and binds it to the PreparedStatement. Again, we're now ready for binding and execute(). In a parse, the vast bulk of the work and locking occurs in preparing an execution plan, so a soft parse is hugely better than a hard parse.
Parse is avoided totally when new parameters are bound to an existing PreparedStatement and it's then executed. No parse is much better than a soft parse.
You only get a soft parse in Oracle when the SQL matches EXACTLY; case, punctuation, and white space all matter. Also, retention in the cache depends on the memory available to Oracle, and the load on the system, in terms of number of different SQL statements being used. This is why it's so important to use parameter binding; "SELECT a FROM b WHERE c = 1;" is not an exact match for "SELECT a FROM b WHERE c = 2"; using ""SELECT a FROM b WHERE c = ?" allows the cache hit to occur.
So coming back to the way you phrased your question: it would be better to use the same PreparedStatement object in both servlets, but because they're tied to a connection, that can be hard to achieve unless you've got a sophisticated database layer between you and the actual database (some of the better connection poolers also do statement caching if configured to do so). A good second best can be creating and using a new instance of PreparedStatement with IDENTICAL sql.
Hope this clarifies...