Commit on each iteration

Question

I have a problem here with transactions and I'm not to sure how to handle this with Java/JDBC.
I have a loop like this:
[begin transaction] setAutoCommit(false);
[select some rows from table a joined with b]
while( there are stil rows ) {
[update table a]
[update table c]
[end transaction] commit(); setAutoCommit(true);
but in this way I lock for myself table "a" for the whole loop and that prevents me from running other instances of the same query which would access other rows in the same table... and worse, if something goes wrong I loose all the processing done so far.
what I want is something like this:
[select some rows from table a joined with b]
while( there are stil rows ) {
[begin transaction] setAutoCommit(false);
[update table a]
[update table c]
[end transaction] commit(); setAutoCommit(true);
but how can I achieve this? Would I have to call connection.createStatement() in each iteration? Would I have to use two separate connections, one for the main loop and one for the updates? I can't figure it out.
Thanks in advance.
Christian.

Answer 1

You can reuse a statement if you don't need previous resultsets obtained from this statement (in case of an instert, update or delete there is no resultset, so your statement can be reused as soon as you executed it).
Fo example you can execute:
Statement stmt = conn.createStatement();
for (int idx = 1; i <= 10; i++) {
    stmt.executeUpdate(
        "UPDATE A SET VALUE = VALUE + 10 WHERE ID = "+idx);
    stmt.executeUpdate(
        "UPDATE B SET VALUE = VALUE - 10 WHERE ID = "+idx);
}but if you will execute the same statements, only changing some values, I recommend you to use PreparedStatements, 'cause they are much faster, for example:
PreparedStatement stmt1 = conn.prepareStatement(
    "UPDATE A SET VALUE = VALUE + 10 WHERE ID = ?");
PreparedStatement stmt2 = conn.prepareStatement(
    "UPDATE B SET VALUE = VALUE + 10 WHERE ID = ?");
for (int idx = 1; i <= 10; i++) {
    stmt1.setInt(1, idx);
    stmt1.executeUpdate();
    stmt2.setInt(1, idx);
    stmt2.executeUpdate();
}both blocks of code do the same thing, but the second is faster. In the first case, the server have to compile 20 sql statements and execute the same number, but in the second case the server only compile 2 sql statements and executes 20. So the second is a lot faster.
rivas.