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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* 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,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.cassandra.cql3.selection;
import java.nio.ByteBuffer;
import java.util.List;
import org.apache.cassandra.config.CFMetaData;
import org.apache.cassandra.cql3.ColumnIdentifier;
import org.apache.cassandra.cql3.ColumnSpecification;
import org.apache.cassandra.cql3.QueryOptions;
import org.apache.cassandra.cql3.functions.Function;
import org.apache.cassandra.cql3.selection.Selection.ResultSetBuilder;
import org.apache.cassandra.db.marshal.AbstractType;
import org.apache.cassandra.exceptions.InvalidRequestException;
import org.apache.cassandra.transport.ProtocolVersion;
/**
* A Selector is used to convert the data returned by the storage engine into the data requested by the
* user. They correspond to the <selector> elements from the select clause.
* Since the introduction of aggregation, Selectors cannot be called anymore by multiple threads
* as they have an internal state.
*/
public abstract class Selector
{
/**
* A factory for Selector instances.
*/
public static abstract class Factory
{
public void addFunctionsTo(List functions)
{
}
/**
* Returns the column specification corresponding to the output value of the selector instances created by
* this factory.
*
* @param cfm the column family meta data
* @return a column specification
*/
public final ColumnSpecification getColumnSpecification(CFMetaData cfm)
{
return new ColumnSpecification(cfm.ksName,
cfm.cfName,
new ColumnIdentifier(getColumnName(), true), // note that the name is not necessarily
// a true column name so we shouldn't intern it
getReturnType());
}
/**
* Creates a new Selector instance.
*
* @param options the options of the query for which the instance is created (some selector
* depends on the bound values in particular).
* @return a new Selector instance
*/
public abstract Selector newInstance(QueryOptions options) throws InvalidRequestException;
/**
* Checks if this factory creates selectors instances that creates aggregates.
*
* @return true if this factory creates selectors instances that creates aggregates,
* false otherwise
*/
public boolean isAggregateSelectorFactory()
{
return false;
}
/**
* Checks if this factory creates writetime selectors instances.
*
* @return true if this factory creates writetime selectors instances,
* false otherwise
*/
public boolean isWritetimeSelectorFactory()
{
return false;
}
/**
* Checks if this factory creates TTL selectors instances.
*
* @return true if this factory creates TTL selectors instances,
* false otherwise
*/
public boolean isTTLSelectorFactory()
{
return false;
}
/**
* Checks if this factory creates Selectors that simply return the specified column.
*
* @param index the column index
* @return true if this factory creates Selectors that simply return
* the specified column, false otherwise.
*/
public boolean isSimpleSelectorFactory(int index)
{
return false;
}
/**
* Returns the name of the column corresponding to the output value of the selector instances created by
* this factory.
*
* @return a column name
*/
protected abstract String getColumnName();
/**
* Returns the type of the values returned by the selector instances created by this factory.
*
* @return the selector output type
*/
protected abstract AbstractType getReturnType();
/**
* Record a mapping between the ColumnDefinitions that are used by the selector
* instances created by this factory and a column in the ResultSet.Metadata
* returned with a query. In most cases, this is likely to be a 1:1 mapping,
* but some selector instances may utilise multiple columns (or none at all)
* to produce a value (i.e. functions).
*
* @param mapping the instance of the column mapping belonging to the current query's Selection
* @param resultsColumn the column in the ResultSet.Metadata to which the ColumnDefinitions used
* by the Selector are to be mapped
*/
protected abstract void addColumnMapping(SelectionColumnMapping mapping, ColumnSpecification resultsColumn);
}
/**
* Add the current value from the specified ResultSetBuilder.
*
* @param protocolVersion protocol version used for serialization
* @param rs the ResultSetBuilder
* @throws InvalidRequestException if a problem occurs while add the input value
*/
public abstract void addInput(ProtocolVersion protocolVersion, ResultSetBuilder rs) throws InvalidRequestException;
/**
* Returns the selector output.
*
* @param protocolVersion protocol version used for serialization
* @return the selector output
* @throws InvalidRequestException if a problem occurs while computing the output value
*/
public abstract ByteBuffer getOutput(ProtocolVersion protocolVersion) throws InvalidRequestException;
/**
* Returns the Selector output type.
*
* @return the Selector output type.
*/
public abstract AbstractType getType();
/**
* Checks if this Selector is creating aggregates.
*
* @return true if this Selector is creating aggregates false
* otherwise.
*/
public boolean isAggregate()
{
return false;
}
/**
* Reset the internal state of this Selector.
*/
public abstract void reset();
}