Package org.apache.cassandra.service.pager

Class AggregationQueryPager

java.lang.Object

org.apache.cassandra.service.pager.AggregationQueryPager

All Implemented Interfaces:

QueryPager

public final class AggregationQueryPager
extends Object
implements QueryPager

QueryPager that takes care of fetching the pages for aggregation queries.

For aggregation/group by queries, the user page size is in number of groups. But each group could be composed of very many rows so to avoid running into OOMs, this pager will page internal queries into sub-pages. So each call to fetchPage(int,org.apache.cassandra.db.ConsistencyLevel,org.apache.cassandra.service.ClientState,org.apache.cassandra.transport.Dispatcher.RequestTime) may (transparently) yield multiple internal queries (sub-pages).

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
final class	AggregationQueryPager.AggregationPartitionIterator	PartitionIterator for queries without Group By but with aggregates.
class	AggregationQueryPager.GroupByPartitionIterator	PartitionIterator that automatically fetch a new sub-page of data if needed when the current iterator is exhausted.

Field Summary

Fields inherited from interface org.apache.cassandra.service.pager.QueryPager

EMPTY

Constructor Summary

Constructors

Constructor	Description
AggregationQueryPager(QueryPager subPager, DataLimits limits)

Method Summary

Modifier and Type	Method	Description
ReadExecutionController	executionController()	Starts a new read operation.
PartitionIterator	fetchPage(int pageSize, ConsistencyLevel consistency, ClientState clientState, Dispatcher.RequestTime requestTime)	Fetches the next page.
PartitionIterator	fetchPageInternal(int pageSize, ReadExecutionController executionController)	Fetches the next page internally (in other, this does a local query).
boolean	isExhausted()	Whether or not this pager is exhausted, i.e.
boolean	isTopK()
int	maxRemaining()	The maximum number of cells/CQL3 row that we may still have to return.
PagingState	state()	Get the current state of the pager.
QueryPager	withUpdatedLimit(DataLimits newLimits)	Creates a new QueryPager that use the new limits.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

AggregationQueryPager

public AggregationQueryPager(QueryPager subPager,
 DataLimits limits)

Method Details

fetchPage

public PartitionIterator fetchPage(int pageSize,
 ConsistencyLevel consistency,
 ClientState clientState,
 Dispatcher.RequestTime requestTime)

Description copied from interface: QueryPager

Fetches the next page.

Specified by:

fetchPage in interface QueryPager

Parameters:

pageSize - the maximum number of elements to return in the next page.

consistency - the consistency level to achieve for the query.

clientState - the ClientState for the query. In practice, this can be null unless consistency is a serial consistency.

Returns:

the page of result.

executionController

public ReadExecutionController executionController()

Description copied from interface: QueryPager

Starts a new read operation.

This must be called before QueryPager.fetchPageInternal(int,org.apache.cassandra.db.ReadExecutionController) and passed to it to protect the read. The returned object must be closed on all path and it is thus strongly advised to use it in a try-with-ressource construction.

Specified by:

executionController in interface QueryPager

Returns:

a newly started order group for this QueryPager.

fetchPageInternal

public PartitionIterator fetchPageInternal(int pageSize,
 ReadExecutionController executionController)

Description copied from interface: QueryPager

Fetches the next page internally (in other, this does a local query).

Specified by:

fetchPageInternal in interface QueryPager

Parameters:

pageSize - the maximum number of elements to return in the next page.

executionController - the ReadExecutionController protecting the read.

Returns:

the page of result.

isExhausted

public boolean isExhausted()

Description copied from interface: QueryPager

Whether or not this pager is exhausted, i.e. whether or not a call to fetchPage may return more result.

Specified by:

isExhausted in interface QueryPager

Returns:

whether the pager is exhausted.

maxRemaining

public int maxRemaining()

Description copied from interface: QueryPager

The maximum number of cells/CQL3 row that we may still have to return. In other words, that's the initial user limit minus what we've already returned (note that it's not how many we *will* return, just the upper limit on it).

Specified by:

maxRemaining in interface QueryPager

state

public PagingState state()

Description copied from interface: QueryPager

Get the current state of the pager. The state can allow to restart the paging on another host from where we are at this point.

Specified by:

state in interface QueryPager

Returns:

the current paging state. Will return null if paging is at the beginning. If the pager is exhausted, the result is undefined.

withUpdatedLimit

public QueryPager withUpdatedLimit(DataLimits newLimits)

Description copied from interface: QueryPager

Creates a new QueryPager that use the new limits.

Specified by:

withUpdatedLimit in interface QueryPager

Parameters:

newLimits - the new limits

Returns:

a new QueryPager that use the new limits

isTopK

public boolean isTopK()

Specified by:

isTopK in interface QueryPager

Returns:

true given read query is a top-k request