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* to you under the Apache License, Version 2.0 (the
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*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
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*/
package org.apache.cassandra.db;
import org.apache.cassandra.db.filter.DataLimits;
import org.apache.cassandra.db.partitions.*;
import org.apache.cassandra.exceptions.RequestExecutionException;
import org.apache.cassandra.service.ClientState;
import org.apache.cassandra.service.pager.QueryPager;
import org.apache.cassandra.service.pager.PagingState;
import org.apache.cassandra.transport.ProtocolVersion;
/**
* Generic abstraction for read queries.
*
* The main implementation of this is {@link ReadCommand} but we have this interface because
* {@link SinglePartitionReadCommand.Group} is also consider as a "read query" but is not a
* {@code ReadCommand}.
*/
public interface ReadQuery
{
ReadQuery EMPTY = new ReadQuery()
{
public ReadExecutionController executionController()
{
return ReadExecutionController.empty();
}
public PartitionIterator execute(ConsistencyLevel consistency, ClientState clientState, long queryStartNanoTime) throws RequestExecutionException
{
return EmptyIterators.partition();
}
public PartitionIterator executeInternal(ReadExecutionController controller)
{
return EmptyIterators.partition();
}
public UnfilteredPartitionIterator executeLocally(ReadExecutionController executionController)
{
return EmptyIterators.unfilteredPartition(executionController.metaData(), false);
}
public DataLimits limits()
{
// What we return here doesn't matter much in practice. However, returning DataLimits.NONE means
// "no particular limit", which makes SelectStatement.execute() take the slightly more complex "paging"
// path. Not a big deal but it's easy enough to return a limit of 0 rows which avoids this.
return DataLimits.cqlLimits(0);
}
public QueryPager getPager(PagingState state, ProtocolVersion protocolVersion)
{
return QueryPager.EMPTY;
}
public boolean selectsKey(DecoratedKey key)
{
return false;
}
public boolean selectsClustering(DecoratedKey key, Clustering clustering)
{
return false;
}
@Override
public boolean selectsFullPartition()
{
return false;
}
};
/**
* Starts a new read operation.
*
* This must be called before {@link executeInternal} 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.
*
* @return a newly started execution controller for this {@code ReadQuery}.
*/
public ReadExecutionController executionController();
/**
* Executes the query at the provided consistency level.
*
* @param consistency the consistency level to achieve for the query.
* @param clientState the {@code ClientState} for the query. In practice, this can be null unless
* {@code consistency} is a serial consistency.
*
* @return the result of the query.
*/
public PartitionIterator execute(ConsistencyLevel consistency, ClientState clientState, long queryStartNanoTime) throws RequestExecutionException;
/**
* Execute the query for internal queries (that is, it basically executes the query locally).
*
* @param controller the {@code ReadExecutionController} protecting the read.
* @return the result of the query.
*/
public PartitionIterator executeInternal(ReadExecutionController controller);
/**
* Execute the query locally. This is similar to {@link ReadQuery#executeInternal(ReadExecutionController)}
* but it returns an unfiltered partition iterator that can be merged later on.
*
* @param controller the {@code ReadExecutionController} protecting the read.
* @return the result of the read query.
*/
public UnfilteredPartitionIterator executeLocally(ReadExecutionController executionController);
/**
* Returns a pager for the query.
*
* @param pagingState the {@code PagingState} to start from if this is a paging continuation. This can be
* {@code null} if this is the start of paging.
* @param protocolVersion the protocol version to use for the paging state of that pager.
*
* @return a pager for the query.
*/
public QueryPager getPager(PagingState pagingState, ProtocolVersion protocolVersion);
/**
* The limits for the query.
*
* @return The limits for the query.
*/
public DataLimits limits();
/**
* @return true if the read query would select the given key, including checks against the row filter, if
* checkRowFilter is true
*/
public boolean selectsKey(DecoratedKey key);
/**
* @return true if the read query would select the given clustering, including checks against the row filter, if
* checkRowFilter is true
*/
public boolean selectsClustering(DecoratedKey key, Clustering clustering);
/**
* Checks if this {@code ReadQuery} selects full partitions, that is it has no filtering on clustering or regular columns.
* @return {@code true} if this {@code ReadQuery} selects full partitions, {@code false} otherwise.
*/
public boolean selectsFullPartition();
}