Chapter 5. Issuing a Query and Processing the Result

Table of Contents

Any time you want to issue SQL statements to the database, you require a Statement or PreparedStatement instance. Once you have a Statement or PreparedStatement, you can use issue a query. This will return a ResultSet instance, which contains the entire result (see the section called “Getting results based on a cursor” here for how to alter this behaviour). Example 5.1, “Processing a Simple Query in JDBC” illustrates this process.

Example 5.1. Processing a Simple Query in JDBC

This example will issue a simple query and print out the first column of each row using a Statement.

Statement st = conn.createStatement();
ResultSet rs = st.executeQuery("SELECT * FROM mytable WHERE columnfoo = 500");
while (rs.next())
{
    System.out.print("Column 1 returned ");
    System.out.println(rs.getString(1));
}
rs.close();
st.close();

This example issues the same query as before but uses a PreparedStatement and a bind value in the query.

int foovalue = 500;
PreparedStatement st = conn.prepareStatement("SELECT * FROM mytable WHERE columnfoo = ?");
st.setInt(1, foovalue);
ResultSet rs = st.executeQuery();
while (rs.next())
{
    System.out.print("Column 1 returned ");
    System.out.println(rs.getString(1));
}
rs.close();
st.close();

Getting results based on a cursor

By default the driver collects all the results for the query at once. This can be inconvenient for large data sets so the JDBC driver provides a means of basing a ResultSet on a database cursor and only fetching a small number of rows.

A small number of rows are cached on the client side of the connection and when exhausted the next block of rows is retrieved by repositioning the cursor.

Note

Cursor based ResultSets cannot be used in all situations. There a number of restrictions which will make the driver silently fall back to fetching the whole ResultSet at once.

  • The connection to the server must be using the V3 protocol. This is the default for (and is only supported by) server versions 7.4 and later.
  • The Connection must not be in autocommit mode. The backend closes cursors at the end of transactions, so in autocommit mode the backend will have closed the cursor before anything can be fetched from it. *The Statement must be created with a ResultSet type of ResultSet.TYPE_FORWARD_ONLY. This is the default, so no code will need to be rewritten to take advantage of this, but it also means that you cannot scroll backwards or otherwise jump around in the ResultSet.
  • The query given must be a single statement, not multiple statements strung together with semicolons.

Example 5.2. Setting fetch size to turn cursors on and off.

Changing code to cursor mode is as simple as setting the fetch size of the Statement to the appropriate size. Setting the fetch size back to 0 will cause all rows to be cached (the default behaviour).

// make sure autocommit is off
conn.setAutoCommit(false);
Statement st = conn.createStatement();

// Turn use of the cursor on.
st.setFetchSize(50);
ResultSet rs = st.executeQuery("SELECT * FROM mytable");
while (rs.next())
{
    System.out.print("a row was returned.");
}
rs.close();

// Turn the cursor off.
st.setFetchSize(0);
rs = st.executeQuery("SELECT * FROM mytable");
while (rs.next())
{
    System.out.print("many rows were returned.");
}
rs.close();

// Close the statement.
st.close();
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