con8643 oracle optimizer bootcamp part2

Oracle Optimizer Bootcamp 10 Optimizer tips you can't do without

Maria Colgan & Jonathan Lewis

Copyright © 2013, Oracle and/or its affiliates. All rights reserved. 2

PART 2


Program Agenda

Using the right tools

Finding the right sample size

Tell the optimizer everything

Functions friends or foe

Most useful hints to know


Tell the Optimizer everything

The Optimizer makes assumptions in certain case

– Each additional where clause predicates reduces the number of rows

– Each additional group by column will increase number of rows returned

– Applying a function to column will alter the NDV of the column

Sometimes you know more about your data than basic statistics show

– Best way to get a good plan is to tell the Optimizer everything

– Otherwise it assumes, making an ass out of u and me


Tell the optimizer everything Correlated column example

SELECT * FROM vehicles WHERE model = ‘530xi’

AND color = 'RED’;

MAKE MODEL COLOR Year

BMW 530xi RED 2013

Cardinality #ROWS * 1 * 1 12 * 1 * 1 1 NDV c1 NDV c2 4 3

= => =


Tell the optimizer everything Correlated column example


AND make = ‘BMW’;


BMW 530xi RED 2013

BMW 530xi BLACK 2011

BMW 530xi BLUE 2012

Cardinality #ROWS * 1 * 1 12 * 1 * 1 1 NDV c1 NDV c2 4 3

=> =


Create extended statistics on column group DBMS_STATS.CREATE_EXTENDED_STATS

New Column with

system generated

name


Tell the optimizer everything Correlated column example with extended statistics


AND make = ‘BMW’;


BMW 530xi RED 2013

BMW 530xi BLACK 2011

BMW 530xi BLUE 2012

Cardinality calculated using column group statistics


Tell the Optimizer everything

Use SEED_COL_USAGE to have Oracle monitor workload

Can monitor STS or live system

Recommends what column groups are needed based on workload

Automatic column group detection


Bad Defaults (a)

create table t1 as

select

decode(

mod(rownum - 1,1000),

0,to_date('31-Dec-4000'),

to_date('01-Jan-2010') +

trunc((rownum-1)/100)

) date_closed

from

large_data_source

where

rownum <= 1827 * 100

;

-- 5 years in days * 100

-- every 1000th row

-- is a 'null' date


Bad Defaults (b)

select * from t1 where date_closed

between to_date('01-Jan-2012','dd-mon-yyyy')

and to_date('31-Dec-2012','dd-mon-yyyy');

-- 36,500 rows

With simple statistics.

TABLE ACCESS (FULL) OF 'T1' (Cost=46 Card=291 Bytes=2328)

With 11 column histogram (two per year and one for the garbage)


With nulls and simple statistics.



Graphical Interpretation

The real data

Oracle's first impression

Impression with histogram

Data for 2012


Bad Data Types (a)

create table t1 as

select

d1,

to_number(to_char(d1,'yyyymmdd')) n1,

to_char(d1,'yyyymmdd') v1

from (

select

to_date('31-Dec-2009') + rownum d1

from all_objects

where

rownum <= 1827 -- 5 years, daily

);

-- date

-- numeric

-- char


Bad Data Types (b)

select * from t1 where d1 -- 7 rows

between to_date('30-Dec-2012','dd-mon-yyyy')

and to_date('05-Jan-2013','dd-mon-yyyy');

On the date type.


N1 between 20121230 and 20030105


V1 between ' 20121230' and '20030105'


10g+ gets Card=397 - everything looks like numbers)


Histogram effects:

N1 between 20121230 and 20130105

– pre-histogram


– post-histogram


V1 between ' 20121230' and '20130105‘

‒ pre-histogram


‒ post-histogram



Graphical Interpretation The real (char/number) data

Oracle's first impression

Impression with histogram


Program Agenda







Functions friends or foe?

Query – How many packages of bounce did we sell?

Sales 2 has a b-tree index on the prod_id column

Expected index range scan but got fast full index scan

SELECT count(*)

FROM sales2

WHERE to_char(prod_id)=‘139’;



Cardinality estimate is in the

right ballpark so not a

problem with statistics

Why is an equality predicate

being evaluated as a filter

and not an access

predicate?

Could it have something to do

with the TO_CHAR function



INTERNAL_FUNCTION indicates Oracle needs to modify predicate

– In this case a data type conversion occurred

Predicate is TO_CHAR(prod_id)

Optimizer has no idea how function effects values in prod_id column

Optimizer can’t determine which rows will be accessed now



What data type is the prod_id column ?

But literal value is a character string ‘139’

Better to apply inverse function on other side of predicate



Query rewrite SELECT count(*)

FROM sales2

WHERE prod_id=to_number(‘139’);

Solution - Use inverse function on other side of predicate



Keep the following in mind when deciding where to place the function

– Try to place functions on top of constants (literals, binds) rather than on columns

– Avoid functions on index columns or partition keys as it prevents index use or

partition pruning

– For function-based index to be considered, use that exact function as specified in

index

– If multiple predicates involve the same columns, write predicates such that they

share common expressions For example,

WHERE f(a) = b WHERE a = inv_f(b)

AND a = c AND a = c

Using inverse function on other side of predicate

Should be

rewritten as

This will allow transitive

predicate c=inv_f(b) to be

added by the optimizer


Virtual Columns (a)

create table t1 as

Select

cast(rownum as number(8,0)) id1,

cast(rownum as number(8,0)) id2

from all_objects

where rownum <= 10000

;

execute dbms_stats.gather_table_stats(user,'t1');

alter table t1 add

m2 generated always as ( mod(id2,3)) virtual

;


Virtual Columns (b)

set autotrace traceonly explain

select * from t1 where mod(id1,3) = 0;

.

|* 1 | TABLE ACCESS FULL| T1 | 100 | 1100 | 4 |

Predicate Information (identified by operation id):

1 - filter(MOD("ID1",3)=0)

select * from t1 where mod(id1,3) != 0;

.



1 - filter(MOD("ID1",3)<>0)


Virtual Columns (c)

set autotrace traceonly explain

select * from t1 where mod(id2,3) = 0;

.



1 - filter("T1"."M2"=0)

select * from t1 where mod(id2,3) != 0;

.



1 - filter("T1"."M2"<>0)


Program Agenda








Understanding Optimizer hints

Hints allow you to influence the Optimizer when it has to choose

between several possibilities

A hint is a directive that will be followed when applicable

Can influence everything from the Optimizer mode used to each

operation in the execution

The hint mechanism is not exclusively used by the Optimizer

Some of the most useful hint influence SQL execution



GATHER_PLAN_STATISTICS triggers execution time statistics to be

preserved

Records actual number of rows seen for each operation (step) in plan

Execution statistis displayed in plan when format parameter of

DMBS_XPLAN set to ALLSTATS LAST

– Means display all execution statistics for last execution of this cursor

GATHER_PLAN_STATISTICS hint


GATHER_PLAN_STATISTICS hint

SELECT * FROM table (

DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS

LAST'));

Compare estimated rows returned for each operation to actual rows


GATHER_PLAN_STATISTICS Hint


DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST'));

Note: a lot of the data is zero in

the A-rows column because we

only show last executed cursor

which is the QC. Need to use

ALLSTATS ALL to see info on all

parallel server cursors


GATHER_PLAN_STATISTICS Hint


DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS ALL'));


Alternatively using MONITOR hint


Most helpful hints to know

Allows value for init.ora Optimizer parameters to be changed for a specific

query

Useful way to prevent setting non-default parameter value system-wide

Only the following Optimizer influencing init.ora parameters can be set:

OPT_PARAM hint

OPTIMIZER_DYNAMIC_SAMPLING

OPTIMIZER_INDEX_CACHING

OPTIMIZER_INDEX_COST_ADJ

OPTIMIZER_USE_PENDING_STATISTICS

Optimizer related underscore parameters

STAR_TRANSFORMATION_ENABLED

PARALLEL_DEGREE_POLICY

PARALLEL_DEGREE_LIMIT


OPT_PARAM hint Example



OPTIMIZER_FEATURES_ENABLE parameter allows you to switch

between optimizer versions

Setting it to previous database version reverts the Optimizer to that

version

– Disables any functionality that was not present in that version

Easy way to work around unexpected behavior in a new release

Hint allows you to revert the Optimizer for just a single statement

OPTIMIZER_FEATURES_ENABLE hint


Precision matters (a)

select /*+ parallel(pt1,2) */

{list of columns}

from pt1

where pt1.n1 = 5

and pt1.n2 = 10

and pt1.pt_group in (0,10)

;

The optimizer DID obey the hint

.

|Id|Operation |Name |Cost |Pstart| Pstop|

| 0|SELECT STATEMENT | | 48| | |

| 1| PARTITION LIST INLIST | | 48|KEY(I)|KEY(I)|

|*2| TABLE ACCESS BY LOCAL INDEX ROWID|PT1 | 48|KEY(I)|KEY(I)|

|*3| INDEX RANGE SCAN |PT1_I1| 3|KEY(I)|KEY(I)|


Precision matters (b)

...

NESTED LOOP

TABLE ACCESS (FULL) OF T7

TABLE ACCESS BY INDEX ROWID T8

INDEX RANGE SCAN T8_IND_SECOND -- “wrong” index

... select /*+ index(t8 t8_ind_first) */ -- hint the “right” one

...

HASH JOIN -- wrong method

TABLE ACCESS (FULL) OF T7

TABLE ACCESS BY INDEX ROWID T8

INDEX FULL SCAN T8_IND_FIRST -- “right” index

...


How much don't we know ? select

/*+

leading(t4 t1 t2 t3)

full(t4)

use_hash(t1) full(t1)



*/

...

From t4, t1, t2, t3

Where t1.id = t4.id1

and t2.id = t4.id2

and t3.id = t4.id3;


How much don't we know ? N – 1 simultaneous build (hash) tables

Maximum two simultaneous build (hash) tables

HASH JOIN

TABLE ACCESS (FULL) OF TABLE_3

HASH JOIN


HASH JOIN



HASH JOIN

HASH JOIN

HASH JOIN






Summary

Forgive the Optimizer – it’s not out to get you

– Its knowledge is limited to the information you provide it

A complete representative set of statistics gets the best plans

– Determine or have Oracle determine what extended statistics are needed

Be mindful of when you capture a representative set of statistics

– Don’t be afraid to adjust maintenance window or include statistics in ETL

Optimizer hints should only be used with extreme caution

– To guarantee the same plan every time use SQL Plan Baselines

Enjoy the challenge!


More Information

White papers

– Optimizer with Oracle Database 12c

– SQL Plan Management with Oracle Database 12c

– Understanding Optimizer Statistics with Oracle Database 12c

Blogs

– http://jonathanlewis.wordpress.com/all-postings/

– http://blogs.oracle.com/optimizer

Oracle.com

– http://www.oracle.com/technetwork/database/focus-areas/bi-

datawarehousing/dbbi-tech-info-optmztn-092214.html

http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-optimizer-with-oracledb-12c-1963236.pdf

http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-sql-plan-mgmt-12c-1963237.pdf

http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-statistics-concepts-12c-1963871.pdf

http://jonathanlewis.wordpress.com/all-postings/



http://blogs.oracle.com/optimizer

http://www.oracle.com/technetwork/database/focus-areas/bi-datawarehousing/dbbi-tech-info-optmztn-092214.html













con8643 oracle optimizer bootcamp part2

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