Understanding Histogram Ppt

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Understanding Histograms in the

Oracle Database

Robert GaydosR Wave [email protected]

Copyright © R WaveSolutions 2008 All rightsreserved



Addenda

What is a Histogram?

Types of Histograms

How to create Histograms How to identify existing Histograms

Rules and tips when creating Histograms

Q & A

Copyright © R Wave Solutions 2008 All rights reserved




A histogram holds the data distribution ofvalues within a column of a table.

–

– Used by CBO to optimize a query.

It is collected by using DBMS_STATS. – By default DBMS_STATS does not collect

histogram stats. Min/Max Value and # of distinct values

NOTE: DBMS_STAT is used to delete histogram data.




Types of Histograms

Two types of Histograms

– frequency histograms

– e g t- a ance stograms – Type of histogram is stored in the HISTOGRAM

column of the *tab_col_statistics views (USER/DBA)

– Value = (‘HEIGHT BALANCED’, ‘FREQUENCY’,or ‘NONE’)




Histogram and Buckets

When Histograms are created the number ofbuckets can be specified.

histogram created.

# Buckets = # of Rows of information.

When Distinct Values <= Buckets – Then Frequency Histogram is created

– Else Height-Balanced Histogram is created




Frequency Histograms

Each value of the column corresponds to asingle bucket of the histogram.

Each bucket contains the number ofoccurrences of that single value.




Frequency Histograms Example

DataABC

SortedABB

ResultsBucket 1 A = 1

BCCCC

CBDE

BCCCC

CCDE

Bucket 2 B = 3Bucket 3 C = 6

Bucket 4 D = 1Bucket 5 E = 1




Height-Balanced Histograms

In a height-balanced histogram, the columnvalues are divided into bands so that each

number of rows.

The useful information that the histogram

provides is where in the range of values theendpoints fall.




Height-Balanced Example 1

DataABC

SortedABB

ResultsEPN EPV

BCCCC

CBDE

BCCCC

CCDE

Buckets = 4

01

23

BC

CE




Height-Balanced Example 1

DataABC

SortedABB

ResultsEPN EPVNotice that C

The DB only stores

0 - B2 - C3 - E

BCCCC

CBDE

BCCCC

CCDE

01

23

Buckets = 4

BC

CE

.

This is howdatabase knowswhich values arepopular




How to Create a Histogram

Created by usingDBMS_STATS.GATHER_TABLE_STATS

= ' _

<# of buckets> <Column Name>‘

execute dbms_stats.gather_table_stats

(ownname => 'oe', tabname => 'inventories',METHOD_OPT => 'FOR COLUMNS SIZE 10quantity_on_hand');




Identify Existing Histograms

In 10g – HISTOGRAM column of the *tab_col_statistics views

(USER/DBA)

In 9select owner, table_name, column_name, count(*)

buckets

from dba_histograms

where endpoint_number not in (0,1)

group by owner,

table_name, column_name




Rules On When To Create Histograms

First there are no rules of thumb that arealways correct.

When creating using dbms_stats, the use ofthe DEGREE (for parallel) can skew the

value of density

Running dbms_stats can drop histograms – Use method_opt==>'REPEAT‘




Rules On When To Create Histograms

Index maintenance overhead is performedduring update, insert and delete operations.

performed during the analyze/dbms_statsprocess.

Histograms are NOT just for indexedcolumns. – Adding a histogram to an un-indexed column that is used in

a where clause can improve performance.




Histogram Example

select sum(t1.d2*t2.d3*t3.d3)from t1, t2, t3where t1.fk1 = t2.pk1

and t3.pk1 = t2.fk1and t3.d2 = 35and t1.d3 = 0; << This column has 2 values in it

In this example column T1.D3 has 2 values and is ourmost selective criterion, but without a histogram CBOassumes an even distribution of 2 distinct values(density = 0.5).




SQL*Plan Without Histogram

Rows Row Source Operation

------- ---------------------------------

1 SORT AGGREGATE

2088 HASH JOIN601 TABLE ACCESS BY INDEX ROWID

601 INDEX RANGE SCAN OBJ#(57612)

1000 HASH JOIN

4 TABLE ACCESS FULL OBJ#(57604)filter("T1"."D3"=0)

62500 TABLE ACCESS FULL OBJ#(57605)




Timing Without Histogram

call count cpu elapsed disk query current rows

------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.00 0.00 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 2 1.04 4.91 19013 20231 0 1

------- ------ -------- ---------- ---------- ---------- ---------- ----------

total 4 1.04 4.91 19013 20231 0 1

Query = 20231




Use DBMS_STATS to Add Histogram

execute dbms_stats.gather_table_stats(NULL,'T1', estimate_percent=>null,

_ =>d3');




Timing With Histogram

call count cpu elapsed disk query current rows

------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.00 0.00 0 0 0 0

Execute 1 0.01 0.00 0 0 0 0

Fetch 2 0.56 2.84 9841 10666 0 1

------- ------ -------- ---------- ---------- ---------- ---------- ----------

total 4 0.57 2.84 9841 10666 0 1

Now Query = 10666

It was Query = 20231

½ the IO




Plan With Histogram

Rows Row Source Operation

------- ---------------------------------------

1 SORT AGGREGATE

601 TABLE ACCESS BY INDEX ROWID OBJ#(57606)601 INDEX RANGE SCAN OBJ#(57612)

1000 TABLE ACCESS BY INDEX ROWID OBJ#(57605)

1005 NESTED LOOPS

4 TABLE ACCESS FULL OBJ#(57604)filter("T1"."D3"=0)

1000 INDEX RANGE SCAN OBJ#(57609)

access("T1"."FK1"="T2"."PK1")




Histograms Opportunities

Any column used in a where clause withskewed data

Columns that are not queried all the time Reduced overhead for insert, update, delete




When Not To Create A Histogram

No rule of thumb is ever perfect.

Do not create Histograms

– Evenly distributed data in the column – Columns that are not queried

– Do not create them on every column of

every table. – PK of a table




Creating / Maintaining Histograms

Table maintenance procedures areimportant.

– cc enta y e et ng stogram ata s poss e.

– Histogram data is ONLY Collected/Maintainedwhen stats are collected (DBMS_STATS).

– Changing data in the table does not change datastored in the histogram.




Maintaining Histograms

Always check if histogram exists on tablebefore DBMS_STATS is run.

– o not orget to note t e # o uc ets.

– Create tables to hold information.

Use METHOD_OPT FOR ALL COLUMNS

REPEAT to prevent deletion of histogramsdata.

Distribution ratio change -> Recollect




Creating Histograms

What is the best way to start out creatingHistograms? Three popular ways to create.

– a t an epper - reate t em as you nee t em.

– Little Hammer DBMS_STATS usingMETHOD_OPT AUTO

Should NOT be used after fresh restarted database

– Big Hammer DBMS_STATS using SKEWONLY

– Over Kill Hammer - FOR ALL COLUMNS




Other Comments

It is the Density of a column that theoptimizer considers when accessing a table.

Changing the number of buckets changesthe density.

More Buckets DOES NOT mean better

density. Using degree != 1 can affect density.




Review


Two types of Histograms in the database

How to create Histograms Data is collected by running DBMS_STATS.

Different methods to create histograms

Bucket size affects density of column.

Density of the column affects CBO.




Thank You for Your Time


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Understanding Histogram Ppt