Oracle Explain Plan

7/29/2019 Oracle Explain Plan

1/52

1


2/52

Explaining the Explain Plan


3/52

Disclaimer

The goal of this session to provide you with a guidefor reading SQL execution plans and to help youdetermine if that plan is what you should be expecting

This session will not provide you with suddenenlightenment making you an Optimizer expert or giveyou the power to tune SQL statements with the flick ofyour wrist!


4/52

Agenda

What is an execution plan and how to generate one

What is a good plan for the optimizer

Understanding execution plans Cardinality

Access paths

Join order

Join type

Partitioning pruning

Parallelism Execution plan examples


5/52

What is an execution

plan and how togenerate one


6/52

What is an Execution plan?

Execution plans show the detailed steps necessary toexecute a SQL statement

These steps are expressed as a set of databaseoperators that consumes and produces rows

The order of the operators and their implementation isdecided by the optimizer using a combination of querytransformations and physical optimization techniques

The display is commonly shown in a tabular format,

but a plan is in fact tree-shaped


7/52

What is an Execution plan?

QuerySELECT prod_category,avg(amount_sold)FROM sales s, products pWHERE p.prod_id = s.prod_idGROUP BY prod_category;

Tabular representation of plan

-----------------------------------------------------------Id Operation Name-----------------------------------------------------------0 SELECT STATEMENT1 HASH GROUP BY2 HASH JOIN3 TABLE ACCESS FULL PRODUCTS4 PARTITION RANGE ALL5 TABLE ACCESS FULL SALES----------------------------------------------------------

Tree-shaped representation of plan

GROUP BY|JOIN_______|_______| |TABLE ACCESS TABLE ACCESSPRODUCTS SALES


8/52

How to get an Execution Plan

Two methods for looking at the execution plan

1.EXPLAIN PLAN command Displays an execution plan for a SQL statement without actually

executing the statement

2.V$SQL_PLAN A dictionary view introduced in Oracle 9i that shows the execution

plan for a SQL statement that has been compiled into a cursor inthe cursor cache

Use DBMS_XPLAN package to display plans

Under certain conditions the plan shown with EXPLAIN PLANcan be different from the plan shown using V$SQL_PLAN


9/52


Example 1 EXPLAIN PLAN command & dbms_xplan.display functionSQL> EXPLAIN PLAN FOR

SELECT prod_category, avg(amount_sold)

FROM sales s, products p

WHERE p.prod_id = s.prod_id

GROUP BY prod_category;

Explained

SQL> SELECT plan_table_outputFROM table(dbms_xplan.display('plan_table',null,'basic'));

------------------------------------------Id Operation Name------------------------------------------

0 SELECT STATEMENT1 HASH GROUP BY2 HASH JOIN3 TABLE ACCESS FULL PRODUCTS4 PARTITION RANGE ALL5 TABLE ACCESS FULL SALES-------------------------------------------


10/52


Example 2 Generate & display execution plan for the last SQL stmtsexecuted in a session

SQL>SELECT prod_category, avg(amount_sold)FROM sales s, products pWHERE p.prod_id = s.prod_idGROUP BY prod_category;

no rows selected

SQL> SELECT plan_table_outputFROM table(dbms_xplan.display_cursor(null,null,'basic'));

------------------------------------------Id Operation Name------------------------------------------0 SELECT STATEMENT1 HASH GROUP BY2 HASH JOIN3 TABLE ACCESS FULL PRODUCTS4 PARTITION RANGE ALL5 TABLE ACCESS FULL SALES-------------------------------------------


11/52


Example 3 Displaying execution plan for any other statementfrom V$SQL_PLAN

1.Directly:

SQL> SELECT plan_table_output FROMtable(dbms_xplan.display_cursor('fnrtqw9c233tt',null,'basic'));

2.Indirectly:SQL> SELECT plan_table_outputFROM v$sql s,TABLE(dbms_xplan.display_cursor(s.sql_id,s.child_number,'basic')) tWHERE s.sql_text like 'select PROD_CATEGORY%';

Note More information on www.optimizermagic.blogspot.com


12/52

What is a good plan

for the optimizer


13/52

Whats a Good Plan for the Optimizer?

The Optimizer has two different goals

Serial execution: Its all about cost The cheaper, the better

Parallel execution: its all about performance The faster, the better

Two fundamental questions:

What is cost?

What is performance?


14/52

What is Cost?

A magically number the optimizer makes up?

Resources required to execute a SQL statement?

Result of complex calculations?

Estimate of how long it will take to execute a statement?

Actual Definition

Cost represents units of work or resources used

Optimizer uses CPU & memory usage plus IO as units of work

Cost is an estimate of the amount of CPU and memory plus thenumber of disk I/Os, used in performing an operation

Cost is an internal Oracle measurement


15/52

What is performance?

Getting as many queries completed as possible?

Getting fastest possible elapsed time using the fewestresources?

Getting the best concurrency rate?

Actual Definition

Performance is fastest possible response time for query

Goal is to complete the query as quickly as possible

Optimizer does not focus on resources needed to execute the plan


16/52

Understanding an

Execution Plan


17/52

SQL Execution Plan

When looking at a plan can you determine if the following iscorrect? Cardinality

Are the correct number of rows coming out of each object?

Access paths

Is the data being accessed in the best way? Scan? Index lookup?

Join order

Are tables being joined in the correct order to eliminate as much data asearly as possible?

Join type

Are the right join types being used?

Partitioning pruning

Did I get partition pruning? Is it eliminating enough data?

Parallelism


18/52

Cardinality

What is it?

Estimate of number rows that will be returned Cardinality for a single value predicate = num_rows total /

num_distinct total E.g. 100 rows total, 10 distinct values => cardinality=10 rows

OR if histogram present num_rows * Density

Why should you care? Influences access method and Join Order

If estimate is off it can have a huge impact on a plan

What causes Cardinality to be wrong? Data Skews

Multiple single column predicates on a table

A function wrapped where clause predicate


19/52

Cardinality or Selectivity

Cardinality the estimated # of rows returned

To determine correct cardinality using a simple SELECT COUNT(*) from eachtable applying any WHERE Clause predicates belonging to that table


20/52

Data Skew

Cardinality = num_rows / num_distinct

If there is a data skew the selectivity could be way off

Create a histogram to correct the selectivity calculation

Oracle automatically creates a histogram if it suspects a data skew

Be careful

Histograms have an interesting side effects on statements with binds

Less relevant for data warehousing

Prior to 11g stmt with binds had only one plan based on first literal value

But presence of a histogram indicate skew unlikely one plan good for allbind values

In 11g multiple execution plans allowed for a single statement


21/52

Multiple Single Column Predicates

Optimizer always assumes each additional predicate

increases the selectivitySelectivity of predicate 1 * selectivity of predicate 2 etc

But real data often shows correlations

Job title influences salary, car model influences make

How do you tell the Optimizer about the correlation?Extended Optimizer Statistics provides a mechanism tocollect statistics on a group of columns

Full integration into existing statistics framework

Automatically maintained with column statistics

Instantaneous and transparent benefit for any migrated application


22/52

A function Wrapped Where Clause

Predicate

SELECT *

FROM customers

WHERE lower(country_id) = 'us';

Applying a function to a column means the optimizer does not know how itwill effect the cardinality

Most likely the optimizer will under-estimate the cardinality

Creating extended statistics for this function allows the optimizer to get thecorrect cardinality

exec dbms_stats.gather_table_stats(sh,'customers', method_opt => -

'for all columns size skewonly for columns(lower(country_id))');


23/52

Access Paths

How to get data out of the table

The access path can be: Full table scan

Table access by Rowid

Index unique scan

Index range scan (descending)

Index skip scan

Full index scan

Fast full index scan

Index joins Bitmap indexes


24/52

Access Path

If you know the wrong access method is being used check cardinality, joinorder

Look in Operation session to seehow obj is being accessed


25/52

Access Path examplesA table countries contains 10K rows & has a primary key oncountry_id What plan would you expect for these queries?Select country_id, name from countries

where country_id in ('AU','FR','IE);

Select country_id, name from countries

where country_id between 'AU' and 'IE';

Select country_id, name from countries where name='USA';


26/52

Join Type

A Join retrieve data from more than one table

Possible join types are

Nested Loops joins

Hash Joins

Partition Wise Joins

Sort Merge joins

Cartesian Joins

Outer Joins


27/52

Join Type Example 1

What Join type should be use for this Query?

SELECT e.name, e.salary, d.dept_nameFROM hr.employees e, hr.departments d

WHERE d.dept_name IN ('Marketing,'Sales')

AND e.department_id=d.department_id;

Employees has 107 rows

Departments has 27 rows

Foreign key relationship between Employees and Departments on dept_id


28/52

Join Type Example 2


SELECT o.customer_id, l.unit_price * l.quantity

FROM oe.orders o ,oe.order_items l

WHERE l.order_id = o.order_id;

Orders has 105 rows

Order Items has 665 rows


29/52

Join Type Example 3


SELECT o.order_id,0.order_date,e.nameFROM oe.orders o , hr.employees e;

Orders has 105 rows



30/52

Join Type Example 4


SELECT d.department_id,e.emp_idFROM hr.employees e FULL OUTER JOIN hr.departments d

ON e.department_id = d.department_id;


Departments has 27 rows

Foreign key relationship between Employees and Departments on dept_id


31/52

Join Type

If the wrong join type is used go back and check the stmt is written correctlyand the cardinality estimates are accurate

Look in the Operation section tocheck the right join type is used


32/52

Join Orders

The order in which the tables are join in a multi table stmt

Ideally start with the table that will eliminate the most rows

Strongly effected by the access paths available

Some basic rules Joins that definitely results in at most one row always go first

When outer joins are used the table with the outer joinoperator must come after the other table in the predicate

If view merging is not possible all tables in the view will bejoined before joining to the tables outside the view


33/52

Join order

If the join order is not correct, check the statistics, cardinality & access methods

1

2

3

Want to start with the table thatreduce the result set the most


34/52

Sales Table

May 22nd 2008

May 23rd 2008

May 24th 2008

May 18th 2008

May 19th 2008

May 20th 2008

May 21st 2008

Select sum(sales_amount)From SALES

Where sales_date between

to_date(05/20/2008,MM/DD/YYYY)

And

to_date(05/23/2008,MM/DD/YYYY);

Q: What was the total

sales for the weekend of

May 20-

22 2008?

Only the 3relevant

partitions are

accessed

Partition Pruning


35/52

Partition Pruning

If you see the word KEY listed it means the partitions touched will be decidedat Run Time

Pstart and Pstop list the partitiontouched by the query


36/52

Bloom Filter

DFO

Hash Join

Filter Create

Receive

DFO

Send

DFO

Send

Scan Time

Receive

Filter Use

Scan Sales

1.Table scan:Time table isscanned and sent

Shared Bloom filter

Set

2.Bloom Filter create:Consumer set creates a

hash table and a BITVECTOR. Bit vectorsets a bit for each rowthat matches the searchconditions

4.Bloom Filter send:BIT VECTOR is sent as

an additional filtercriteria to the scan of thesales table

3.Table Scan:Sales table is scanand rows are filteredbased on querypredicates

6.Reduced row sent:Only rows that have amatch in the bit vectorget sent to theconsumers

Test

5.Bloom Filter apply:Join column is hashedand compared to BITVECTOR

7.Hash Join:Consumers will complete the hash

join by probing into the hash tablefrom the time time to find actualmatching rows


37/52

Parallelism

Goal is to execute all aspects of the plan in parallel Identify if one or more sets of parallel server processes are used

Producers and Consumers

Identify if any part of the plan is running serial


38/52

Parallelism

If you see any lines beginning with the letter S you are running Serial checkDOP for each table & index used

IN-OUT column shows whichstep is run in parallel and if it is asingle parallel server set or not


39/52

Identifying Granules of Parallelism

during scans in the plan

Data is Partitioned into Granules either block range

Partition

Each parallel server is allocated one or more granules

The granule method is specified on line above thescan in the operation section


40/52

Identifying Granules of Parallelism

during scans in the plan


41/52

Access Paths and how they are

parallelized

Access Paths Parallelization method

Full table scan Block Iterator

Table accessed by Rowid Partition

Index unique scan PartitionIndex range scan (descending) Partition

Index skip scan Partition

Full index scan Partition

Fast full index scan Block IteratorBitmap indexes (in Star Transformation) Block Iterator


42/52

Parallel Distribution

Necessary when producers & consumers sets are used

Producers must pass or distribute their data intoconsumers

Operator into which the rows flow decides the distribution

Distribution can be local or across other nodes in RAC

Five common types of redistribution


43/52


HASH Assumes one of the tables is hash partitioned

Hash function applied to value of the join column

Distribute to the consumer working on the corresponding hash partition

Broadcast The size of one of the result sets is small

Sends a copy of the data to all consumers

Range Typically used for parallel sort operations Individual parallel servers work on data ranges

QC doesnt have to sort just present the parallel server results in the correct order

Partitioning Key Distribution PART (KEY) Assumes that the target table is partitioned

Partitions of the target tables are mapped to the parallel servers Producers will map each scanned row to a consumer based on the partitioning column

Round Robin Randomly but evenly distributes the data among the consumers


44/52


Shows how the PQ serversdistribute rows betweeneach other


45/52

Example of reading a

plan


46/52

Example SQL Statement and Block

DiagramSELECT '(' || pcode || ')' || pcode_desc AS PRODUCT, CNT

FROM (SELECT a.pcode, b.pcode_desc, count(a.pcode) CNT

FROM BMG.t_acct_master_hd a

,BMG.hogan_pcode_hd_ref b

,BMG.t_tran_detail_hd c

WHERE a.pcode = b.pcode

AND a.acct_num=c.acct_numAND a.co_id=c.co_id

AND c.asof_yyyymm=200102

AND c.tran_amt


47/52

Example Cont d Execution plan

2. Are the cardinalityestimates correct?

1. Check the rows returned isapprox correct

Means no stats gatheredstrong indicate this wontbe best possible plan

3.Are the access method correct?

Example Contd Execution plan


48/52


4. Has partitionpruning happen?

5. Are the correct join methods used?

1

2

3

6. Is the join order correct? Is the table thateliminates the most rows accessed first?

Example Contd Execution plan


49/52


7. Check all aspects of the plan are executing inparallel

8. Check the distribution method and make sure weare not broadcasting a large table?

Example Contd Execution plan Solution


50/52

Example Cont d Execution plan - Solution

4. Partitionpruning Onerangepartition 4hashpartitions

8. Rowdistributionis now allhash

2. Cardinalitiesare correct andwith each joinnumber of rowsreduced

1. Only 1 row is actually returnedand the cost is 4 X lower now

5. Join types are stillhash joins but now aPWJ

1

2

3

6. The join order has changed - PWJthem join hash to look-up table

3. Access methodsremains the same

7. All aspectsof the plan areexecuting inparallel

D t i i if t th i ht l


51/52

Determining if you get the right plan

Query

SELECT quantity_soldFROM sales s, customers cWHERE s.cust_id =c.cust_id;ID

What do you expect the plan to look like for this statement?S NOT NULL)

Explanation

Join to customers is redundant as no columns are selected

Presence of primaryforeign key relationship means we can remove table

Q & A


52/52

Q & A

Oracle Explain Plan

Documents

Transcript of Oracle Explain Plan