In Java, how does PreparedStatement work for the following query

Question

I have a query like the following and was wondering what kind of SQL is produced by batching a PreparedStatement.

INSERT INTO table1 (id, version, data)
  VALUES (?, ?, ?)
  ON DUPLICATE KEY UPDATE 
    table1.data = IF(table1.version > table2.version, table1.data, table2.data),
    table1.version = IF(table1.version > table2.version, table1.version, table2.version)

The question is, will it resolve this to a copy of this whole sql string for each row in the batch or will it do something like:

INSERT INTO table1 (id, version, data)
  VALUES (a1, b1, c1), (a2, b2, c2), (a3, b3, c3), ...
  ON DUPLICATE KEY UPDATE 
    table1.data = IF(table1.version > table2.version, table1.data, table2.data),
    table1.version = IF(table1.version > table2.version, table1.version, table2.version)

If not, what is the performance implication and how do I write it in such a way that I can batch many of these INSERT..UPDATE statements using PreparedStatement without incurring a performance penalty?

Answer 1

A prepared statement just inserts the positional values you put in into a repeating statement which then doesn't need to be parsed each time. So your second form would just require N * 3 parameters and wouldn't give you any of the speed improvement of a prepared statement. For repeating statements you want to use addTobatch. Basically you prepare the statement, (e.g. "UPDATE ... ? ? ? " and then add 3 parameters at a time, and execute the batch all at once.

I used to use something like this as a utility to wrap the messiness of this. So you'd just do something like

  SQLBatchHandler h = new SQLBatchHandler(conn, "UPDATE ... WHERE ? ? ? ... ");
  h.addToBatch(x, y,z);
  h.addToBatch(x2,y2,z2);
  ...
  h.flush();



public class SQLBatchHandler {
    public static int           MAX_BATCH_SIZE  = 500;
    public String           query;
    private Connection      conn;
    private PreparedStatement   ps;
    private int             batch_ct;

    public SQLBatchHandler(Connection c, String query) throws SQLException
        {
        conn = c;
        this.query = query;
        ps = conn.prepareStatement(query);
    }

    /**
     * add this row to the batch and handle the commit if the batch size
     * exceeds {@link #MAX_BATCH_SIZE}
     * 
     * @param values row values
     * @throws SQLException
     */
    public void addToBatch(Object ... values) throws SQLException
    {
        int i = 0;
        for (Object value: values)
        {
            ps.setObject((++i), value);
        }
        add();
    }

    private void add() throws SQLException
    {
        ps.addBatch();
        if ((++batch_ct) > MAX_BATCH_SIZE)
        {
            ps.executeBatch();
            batch_ct = 0;
        }
    }

    /**
     * Commit any remaining objects and close.
     * 
     * @throws SQLException On statement close error.
     */
    public void flush() throws SQLException
    {
        if (batch_ct == 0) { return; }
        try
        {
            ps.executeBatch();
        }
        catch (SQLException e)
        {
            throw e;
        }
        finally
        {
            if (ps != null)
            {
                ps.close();
            }
        }
    }
}