Lock database

Question

Hi all,
I have the client/server architecture in my project, where the java application is in the client and dbms is in the server. So, its possible that n clients are connected to the database, all with its own connection. The tables in database use only auto incremented primary keys. Suppose the primary keys are all named "id". So, when I insert a row, I need to know what is the id of the inserted row, right after the insertion. But I need to somehow lock the database, to guarantee that my insert statement is followed by my query to get the current value of the sequence in the id (the value representing the id of the inserted row). I must do this because in my understand of things, another client can insert a row in between the insertion and retrieval of the id in another client. Then, if my understanding is correct, the client will retrieval the wrong value for the id.
In JDBC, I can set auto commit to false, then with a statement object execute the insertion and query the id, and after that commiting, but I don't know if this locks the database.
Anyone can help me identify a way to guarantee that two sql statements are executed in sequence in the database using the JDBC?
I am using Postgresql

Answer 1

The sequence is good per transaction. From the Postgres docs: http://www.postgresql.org/docs/8.2/interactive/functions-sequence.html
Return the value most recently obtained by nextval for this sequence in the current session. (An error is reported if nextval has never been called for this sequence in this session.) Notice that because this is returning a session-local value, it gives a predictable answer whether or not other sessions have executed nextval since the current session did.
So, you should be fine. It's good practice, as alluded to, to perform these operations all in the same transaction.
- Saish