| PostgreSQL 7.4.3 Documentation | ||||
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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 Section 31.3.1
here for how to alter this behaviour).
Example 31-1 illustrates this process.
Example 31-1. 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 = db.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 = db.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();
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.
Example 31-2. 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).
Statement st = db.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);
ResultSet rs = st.executeQuery("SELECT * FROM mytable");
while (rs.next()) {
System.out.print("many rows were returned.");
}
rs.close();
// Close the statement.
st.close();
Statement or PreparedStatement Interface The following must be considered when using the
Statement or
PreparedStatement interface:
You can use a single Statement instance
as many times as you want. You could create one as soon as you
open the connection and use it for the connection's
lifetime. But you have to remember that only one
ResultSet can exist per
Statement or
PreparedStatement at a given time.
If you need to perform a query while processing a
ResultSet, you can simply create and
use another Statement.
If you are using threads, and several are using the database,
you must use a separate Statement for
each thread. Refer to Section 31.9 if you are
thinking of using threads, as it covers some important points.
When you are done using the Statement
or PreparedStatement
you should close it.
ResultSet Interface The following must be considered when using the
ResultSet interface:
Before reading any values, you must call
next(). This returns true if there is a
result, but more importantly, it prepares the row for
processing.
Under the JDBC specification, you should access a field only once. It is safest to stick to this rule, although at the current time, the PostgreSQL driver will allow you to access a field as many times as you want.
You must close a ResultSet by calling
close() once you have finished using it.
Once you make another query with the
Statement used to create a
ResultSet, the currently open
ResultSet instance is closed
automatically.