== Cursors and streaming

include::cursor_warning.adoc[opts=optional]

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 age > @p1")
  .onComplete(ar1 -> {
  if (ar1.succeeded()) {
    PreparedStatement pq = ar1.result();

    // Create a cursor
    Cursor cursor = pq.cursor(Tuple.of(18));

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

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

Cursors shall be closed when they are released prematurely:

[source,java]
----
cursor
  .read(50)
  .onComplete(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 age > @p1")
  .onComplete(ar1 -> {
  if (ar1.succeeded()) {
    PreparedStatement pq = ar1.result();

    // Fetch 50 rows at a time
    RowStream<Row> stream = pq.createStream(50, Tuple.of(18));

    // Use the stream
    stream.exceptionHandler(err -> {
      System.out.println("Error: " + err.getMessage());
    });
    stream.endHandler(v -> {
      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.