== Cursors and streaming

By default prepared query execution fetches all rows, you can use a
`link:../../apidocs/io/vertx/sqlclient/Cursor.html[Cursor]` to control the amount of rows you want to read:

[source,java]
----
connection.prepare("SELECT * FROM users WHERE first_name LIKE $1", ar1 -> {
  if (ar1.succeeded()) {
    PreparedQuery pq = ar1.result();

    // Cursors require to run within a transaction
    Transaction tx = connection.begin();

    // Create a cursor
    Cursor cursor = pq.cursor(Tuple.of("julien"));

    // Read 50 rows
    cursor.read(50, ar2 -> {
      if (ar2.succeeded()) {
        RowSet rows = ar2.result();

        // Check for more ?
        if (cursor.hasMore()) {
          // Repeat the process...
        } else {
          // No more rows - commit the transaction
          tx.commit();
        }
      }
    });
  }
});
----

PostreSQL destroys cursors at the end of a transaction, so the cursor API shall be used
within a transaction, otherwise you will likely get the `34000` PostgreSQL error.

Cursors shall be closed when they are released prematurely:

[source,java]
----
cursor.read(50, ar2 -> {
  if (ar2.succeeded()) {
    // Close the cursor
    cursor.close();
  }
});
----

A stream API is also available for cursors, which can be more convenient, specially with the Rxified version.

[source,java]
----
connection.prepare("SELECT * FROM users WHERE first_name LIKE $1", ar1 -> {
  if (ar1.succeeded()) {
    PreparedQuery pq = ar1.result();

    // Streams require to run within a transaction
    Transaction tx = connection.begin();

    // Fetch 50 rows at a time
    RowStream<Row> stream = pq.createStream(50, Tuple.of("julien"));

    // Use the stream
    stream.exceptionHandler(err -> {
      System.out.println("Error: " + err.getMessage());
    });
    stream.endHandler(v -> {
      tx.commit();
      System.out.println("End of stream");
    });
    stream.handler(row -> {
      System.out.println("User: " + row.getString("last_name"));
    });
  }
});
----

The stream read the rows by batch of `50` and stream them, when the rows have been passed to the handler,
a new batch of `50` is read and so on.

The stream can be resumed or paused, the loaded rows will remain in memory until they are delivered and the cursor
will stop iterating.