com.persistit

Class KeyHistogram

java.lang.Object

com.persistit.KeyHistogram

public class KeyHistogram
extends Object

Accumulate and hold information about the distribution of Key objects in a Persistit Tree. This class is used by the Exchange.computeHistogram(Key, Key, int, int, KeyFilter, int) method to accumulate and return the result of scanning all the keys at a fixed depth within a Tree.

The result is represented by a List of KeyHistogram.KeyCount objects, each representing a key and a count. The count represents the number of smaller keys in the tree level. With this information client applications can estimate the number of elements between any two keys in the Tree.

Application code can specify a keyDepth at which sibling keys are grouped together. For example, suppose a Tree contains keys such as

   {"BLUE",1}
   {"BLUE",2}
   {"BLUE",3}
   {"RED",1}
   {"RED",2}
 

If keyDepth=2 the resulting histogram will have 5 buckets, each with a count value of 1. But if keyDepth=1 the result will have two buckets, one each for "BLUE" and "RED". Specifying keyDepth=0 turns off aggregation by partial key depth.

During the aggregation process the Exchange.computeHistogram(com.persistit.Key, com.persistit.Key, int, int, com.persistit.KeyFilter, int) method invokes the addKeyCopy(com.persistit.Key) method for each Key it traverses. Each key is analyzed to determine whether the first keyDepth segments are the same as the previously added Key. If so then the previous count is incremented; otherwise a new KeyCount entry is added to the sample list.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	KeyHistogram.KeyCount Element in a KeyHistogram that denotes the estimated number of keys in a histogram bucket.

Constructor Summary

Constructors

Constructor and Description
KeyHistogram(Tree tree, Key start, Key end, int sampleSize, int keyDepth, int treeDepth)

Method Summary

Modifier and Type	Method and Description
Key	getEndKey()
int	getKeyCount()
int	getKeyDepth()
long	getPageBytesInUse()
long	getPageBytesTotal()
int	getPageCount()
int	getRequestedSampleSize()
List<KeyHistogram.KeyCount>	getSamples()
int	getSampleSize()
Key	getStartKey()
Tree	getTree()
int	getTreeDepth()

Methods inherited from class java.lang.Object

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

Constructor Detail

KeyHistogram

public KeyHistogram(Tree tree,
                    Key start,
                    Key end,
                    int sampleSize,
                    int keyDepth,
                    int treeDepth)

Method Detail

getTree

public Tree getTree()

getStartKey

public Key getStartKey()

getEndKey

public Key getEndKey()

getKeyCount

public int getKeyCount()

getRequestedSampleSize

public int getRequestedSampleSize()

getSampleSize

public int getSampleSize()

getSamples

public List<KeyHistogram.KeyCount> getSamples()

getTreeDepth

public int getTreeDepth()

getKeyDepth

public int getKeyDepth()

getPageCount

public int getPageCount()

getPageBytesTotal

public long getPageBytesTotal()

getPageBytesInUse

public long getPageBytesInUse()