77 lines
2.8 KiB
ReStructuredText
77 lines
2.8 KiB
ReStructuredText
.. _query-paging:
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Paging Large Queries
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====================
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Cassandra 2.0+ offers support for automatic query paging. Starting with
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version 2.0 of the driver, if :attr:`~.Cluster.protocol_version` is greater than
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:const:`2` (it is by default), queries returning large result sets will be
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automatically paged.
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Controlling the Page Size
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-------------------------
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By default, :attr:`.Session.default_fetch_size` controls how many rows will
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be fetched per page. This can be overridden per-query by setting
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:attr:`~.fetch_size` on a :class:`~.Statement`. By default, each page
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will contain at most 5000 rows.
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Handling Paged Results
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----------------------
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Whenever the number of result rows for are query exceed the page size, an
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instance of :class:`~.PagedResult` will be returned instead of a normal
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list. This class implements the iterator interface, so you can treat
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it like a normal iterator over rows::
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from cassandra.query import SimpleStatement
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query = "SELECT * FROM users" # users contains 100 rows
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statement = SimpleStatement(query, fetch_size=10)
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for user_row in session.execute(statement):
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process_user(user_row)
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Whenever there are no more rows in the current page, the next page will
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be fetched transparently. However, note that it *is* possible for
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an :class:`Exception` to be raised while fetching the next page, just
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like you might see on a normal call to ``session.execute()``.
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If you use :meth:`.Session.execute_async()` along with,
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:meth:`.ResponseFuture.result()`, the first page will be fetched before
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:meth:`~.ResponseFuture.result()` returns, but latter pages will be
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transparently fetched synchronously while iterating the result.
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Handling Paged Results with Callbacks
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-------------------------------------
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If callbacks are attached to a query that returns a paged result,
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the callback will be called once per page with a normal list of rows.
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Use :attr:`.ResponseFuture.has_more_pages` and
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:meth:`.ResponseFuture.start_fetching_next_page()` to continue fetching
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pages. For example::
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class PagedResultHandler(object):
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def __init__(self, future):
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self.error = None
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self.finished_event = Event()
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self.future = future
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self.future.add_callbacks(
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callback=self.handle_page,
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errback=self.handle_err)
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def handle_page(self, rows):
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for row in rows:
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process_row(row)
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if self.future.has_more_pages:
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self.future.start_fetching_next_page()
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else:
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self.finished_event.set()
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def handle_error(self, exc):
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self.error = exc
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self.finished_event.set()
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future = session.execute_async("SELECT * FROM users")
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handler = PagedResultHandler(future)
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handler.finished_event.wait()
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if handler.error:
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raise handler.error
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