== 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,groovy]
----
connection.prepare("SELECT * FROM users WHERE age > ?", { ar1 ->
  if (ar1.succeeded()) {
    def pq = ar1.result()

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

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

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

----

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,groovy]
----
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,groovy]
----
connection.prepare("SELECT * FROM users WHERE age > ?", { ar1 ->
  if (ar1.succeeded()) {
    def pq = ar1.result()

    // Fetch 50 rows at a time
    def stream = pq.createStream(50, Tuple.of(18))

    // Use the stream
    stream.exceptionHandler({ err ->
      println("Error: ${err.getMessage()}")
    })
    stream.endHandler({ v ->
      println("End of stream")
    })
    stream.handler({ row ->
      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.