1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one
3 * or more contributor license agreements. See the NOTICE file
4 * distributed with this work for additional information
5 * regarding copyright ownership. The ASF licenses this file
6 * to you under the Apache License, Version 2.0 (the
7 * "License"); you may not use this file except in compliance
8 * with the License. You may obtain a copy of the License at
9 *
10 * http://www.apache.org/licenses/LICENSE-2.0
11 *
12 * Unless required by applicable law or agreed to in writing,
13 * software distributed under the License is distributed on an
14 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
15 * KIND, either express or implied. See the License for the
16 * specific language governing permissions and limitations
17 * under the License.
18 *
19 */
20 package org.apache.directory.server.xdbm.search.impl;
21
22
23 import java.util.Comparator;
24
25 import org.apache.directory.server.xdbm.search.Evaluator;
26
27
28 /**
29 * A helper class used to compare scan counts.
30 *
31 * @author <a href="mailto:dev@directory.apache.org">Apache Directory Project</a>
32 */
33 public class ScanCountComparator implements Comparator<Evaluator<?>>
34 {
35 /**
36 * Compare two scan counts frpm two evaluators
37 */
38 public int compare( Evaluator<?> e1, Evaluator<?> e2 )
39 {
40 Object count1 = e1.getExpression().get( DefaultOptimizer.COUNT_ANNOTATION );
41 Object count2 = e2.getExpression().get( DefaultOptimizer.COUNT_ANNOTATION );
42 long scanCount1 = Long.MAX_VALUE;
43 long scanCount2 = Long.MAX_VALUE;
44
45 if ( count1 != null )
46 {
47 scanCount1 = ( Long ) e1.getExpression().get( DefaultOptimizer.COUNT_ANNOTATION );
48 }
49
50 if ( count2 != null )
51 {
52 scanCount2 = ( Long ) e2.getExpression().get( DefaultOptimizer.COUNT_ANNOTATION );
53 }
54
55 if ( scanCount1 == scanCount2 )
56 {
57 return 0;
58 }
59
60 /*
61 * We want the Evaluator with the smallest scan count first
62 * since this node has the highest probability of failing, or
63 * rather the least probability of succeeding. That way we
64 * can short the sub-expression evaluation process.
65 */
66 if ( scanCount1 < scanCount2 )
67 {
68 return -1;
69 }
70
71 return 1;
72 }
73 }