statisticsimpl.java

来自「derby database source code.good for you.」· Java 代码 · 共 123 行

/*

   Derby - Class org.apache.derby.catalog.types.StatisticsImpl

   Copyright 2001, 2004 The Apache Software Foundation or its licensors, as applicable.

   Licensed 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.derby.catalog.types;

import org.apache.derby.catalog.Statistics;
import org.apache.derby.iapi.services.io.Formatable;
import org.apache.derby.iapi.services.io.StoredFormatIds;
import org.apache.derby.iapi.services.io.FormatableHashtable;
import org.apache.derby.iapi.services.io.FormatableLongHolder;

import java.io.ObjectOutput;
import java.io.ObjectInput;
import java.io.IOException;

public class StatisticsImpl	implements Statistics, Formatable
{
	/* total count of rows for which this statistic was created-- this
	   is not the same as the total number of rows in the conglomerate
	   currently, but a snapshot; i.e the  number of rows when this
	   statistic was created/updated.
	*/

	private long numRows;
	
	/* total count of unique values for the keys 
	 */
	private long numUnique;

	/**
	 * Constructor for StatisticsImpl.
	 * 
	 * @param numRows	number of rows in the conglomerate for which
	 * this statistic is being created.
	 * @param numUnique number of unique values in the key for which
	 * this statistic is being created.
	 */
	public StatisticsImpl(long numRows, long numUnique)
	{
		this.numRows = numRows;
		this.numUnique = numUnique;
	}

	/** Zero argument constructor for Formatable Interface */
	public StatisticsImpl()
	{}

	/** @see Statistics#selectivity */
	public double selectivity(Object[] predicates)
	{
		if (numRows == 0.0)
			return 0.1;

		/* xxxSTATresolve: for small values of numRows, should we do something
		 * special? 
		 */
		return (double)(1/(double)numUnique);
	}

	/*------------------ Externalizable Interface ------------------*/
	
	/**
	 * @see java.io.Externalizable#readExternal
	 */
	public void readExternal(ObjectInput in) 
		throws IOException, ClassNotFoundException
	{
		FormatableHashtable fh = (FormatableHashtable)in.readObject();
		numRows = fh.getLong("numRows");
		numUnique = fh.getLong("numUnique");
	}

	/**
	 * Write this object to a stream of stored objects.
	 *
	 * @param out write bytes here.
	 *
	 * @exception IOException		thrown on error
	 */
	public void writeExternal(ObjectOutput out)
		 throws IOException
	{
		FormatableHashtable fh = new FormatableHashtable();
		fh.putLong("numRows", numRows);
		fh.putLong("numUnique", numUnique);
		out.writeObject(fh);
	}
		
	/*------------------- Formatable Interface ------------------*/
	/**
	 * @return the format id which corresponds to this class.
	 */
	public int getTypeFormatId()
	{
		return StoredFormatIds.STATISTICS_IMPL_V01_ID;
	}

	
	/** @see java.lang.Object#toString */
	public String toString()
	{
		return "numunique= " + numUnique + " numrows= " + numRows;
	}
	
}