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Explaining the Explain Plan
Maria Colgan
Master Product Manager
Oracle Database Server Technologies
June 2018
Interpreting Execution Plans for SQL Statements
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Safe Harbor Statement
The following is intended to outline our general product direction. It is intended for
information purposes only, and may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality, and should not be relied upon
in making purchasing decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole discretion of Oracle.
2
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Agenda
What is an execution plan
How to generate a plan
What is a good plan for the Optimizer
Understanding execution plans
Execution plan examples
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What is an execution plan?
• Execution plans show the detailed steps necessary to execute a SQL statement
• These steps are expressed as a set of database operators that consumes and
produces rows
• The order of the operators and their implementation is decided by the
optimizer using a combination of query transformations and physical
optimization techniques
• The display is commonly shown in a tabular format, but a plan is in fact tree-
shaped
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What is an execution plan?
Query:
SELECT prod_category, avg(amount_sold)
FROM sales s, products p
WHERE p.prod_id = s.prod_id
GROUP BY prod_category;
HASH JOIN
TABLE ACCESS
SALES
Tree-shaped representation of plan
TABLE ACCESS
PRODUCTS
GROUP BY
Tabular representation of plan
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Agenda
What is an execution plan
How to generate a plan
What is a good plan for the Optimizer
Understanding execution plans
Execution plan examples
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
• Autotrace • SQL Monitor
7
• SQL Developer • TKPROF
Many Ways to View an Execution Plan
…..But there are actually only 2 ways to generate one
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
How to generate an execution plan
1.EXPLAIN PLAN command
– Displays an execution plan for a SQL statement without actually executing the
statement
2.V$SQL_PLAN
– A dictionary view introduced in Oracle 9i that shows the execution plan for a SQL
statement that has been compiled into a cursor in the cursor cache
Two methods for looking at the execution plan
Under certain conditions the plan shown with EXPLAIN PLAN can be
different from the plan shown using V$SQL_PLAN
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
How to generate an execution plan
SQL> EXPLAIN PLAN FOR
SELECT p.prod_name, avg(s.amount_sold)
FROM sales s, products p
WHERE p.prod_id = s.prod_id
GROUP BY p.prod_name;
SQL> SELECT * FROM
table(dbms_xplan.display('plan_table',null,'basic'));
EXPLAIN PLAN command & dbms_xplan.display function
PLAN TABLE
NAME
STATEMENT
ID
FORMAT
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
How to generate an execution plan
SQL> SELECT p.prod_name, avg(s.amount_sold)
FROM sales s, products p
WHERE p.prod_id = s.prod_id
GROUP BY p.prod_name;
SQL> SELECT * FROM
table(dbms_xplan.display_cursor(null, null, 'basic'));
Generate & display execution plan for last SQL statements executed in session
SQL_ID CHILD
NUMBER
FORMAT
• Format* is highly customizable - Basic ,Typical, All
– Additional low level parameters show more detail *More information on formatting on Optimizer blog
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Agenda
What is an execution plan
How to generate a plan
What is a good plan for the Optimizer
Understanding execution plans
Execution plan examples
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What’s a good plan for the Optimizer?
The Optimizer has two different goals
• Serial execution: It’s all about cost
– The cheaper, the better
• Parallel execution: it’s all about performance
– The faster, the better
Two fundamental questions:
• What is cost
• What is performance
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What is cost?
• A magically number the optimizer makes up?
• Resources required to execute a SQL statement?
• Estimate of how long it will take to execute a statement?
Actual Definition
• Cost represents units of work or resources used
• Optimizer uses CPU & IO as units of work
• Estimate of amount of CPU & disk I/Os, used to perform an operation
Cost is an internal Oracle measurement & should be used for comparison purposes only
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What is performance?
• Getting as many queries completed as possible?
• Getting fastest possible elapsed time using the fewest resources?
• Getting the best concurrency rate?
Actual Definition
• Performance is fastest possible response time for a query
• Goal is to complete the query as quickly as possible
• Optimizer does not focus on resources needed to execute the plan
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Agenda
What is an execution plan
How to generate a plan
What is a good plan for the optimizer
Understanding execution plans
• Cardinality
• Access paths
• Join methods
• Join order
Execution plan examples
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Cardinality
What is it?
• Estimate of number rows that will be returned by each operation
How does the Optimizer Determine it?
• Cardinality for a single column equality predicate = total num of rows
num of distinct values
– For example: A table has 100 rows, a column has 5 distinct values
=> cardinality=20 rows
• More complicated predicates have more complicated cardinality calculation
Why should you care?
It influences everything! Access method, Join type, Join Order etc
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Identifying cardinality in an execution plan
Cardinality - estimated # of
rows returned
Determine correct cardinality using a SELECT
COUNT(*) from each table applying any WHERE
Clause predicates belonging to that table
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Checking cardinality estimates
SELECT /*+ gather_plan_statistics */
p.prod_name, SUM(s.quantity_sold)
FROM sales s, products p
WHERE s.prod_id =p.prod_id
GROUP BY p.prod_name ;
SELECT * FROM table (
DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST'));
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Checking cardinality estimates
SELECT * FROM table (
DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST'));
Compare estimated number of rows (E-Rows) with actual rows returned (A-Rows)
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Checking cardinality estimates
SELECT * FROM table (
DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST'));
Extra information you get with ALLSTATS
Starts indicates the number of times that step or operation was done
In this case the SALES table is partitioned and has 28 partitions
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Checking cardinality estimates
SELECT * FROM table (
DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST'));
Extra information you get with ALLSTATS
Buffers indicates the number of buffers that need to be read for each step
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Checking cardinality estimates
SELECT * FROM table (
DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST'));
Extra information you get with ALLSTATS
0Mem - estimated amount of memory needed
1Mem - amount of memory need to perform the operation in 1 pass
Used-Mem - actual amount of memory used and number of passes required
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Checking cardinality estimates for Parallel Execution
SELECT * FROM table (
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
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Checking cardinality estimates for Parallel Execution
SELECT * FROM table (
DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS ALL'));
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Easiest way to compare the estimated number of rows returned with actual rows returned
Check cardinality using SQL Monitor
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Solutions to incorrect cardinality estimates
Cause Solution
Stale or missing statistics DBMS_STATS
Data Skew Create a histogram
Multiple single column predicates on a table Create a column group using
DBMS_STATS.CREATE_EXTENDED_STATS
Function wrapped column Create statistics on the funct wrapped column
using DBMS_STATS.CREATE_EXTENDED_STATS
Multiple columns used in a join Create a column group on join columns using
DBMS_STATS.CREATE_EXTENDED_STAT
Complicated expression containing columns
from multiple tables
Use dynamic sampling level 4 or higher
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Agenda
What is an execution plan
How to generate a plan
What is a good plan for the optimizer
Understanding execution plans
• Cardinality
• Access paths
• Join methods
• Join order
Execution plan examples
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Access paths – Getting the data
Access Path Explanation
Full table scan Reads all rows from table & filters out those that do not meet the where clause predicates. Used when no index,
DOP set etc
Table access by Rowid Rowid specifies the datafile & data block containing the row and the location of the row in that block. Used if
rowid supplied by index or in where clause
Index unique scan Only one row will be returned. Used when table contains a UNIQUE or a PRIMARY KEY constraint that
guarantees that only a single row is accessed
Index range scan Accesses adjacent index entries returns ROWID values Used with equality on non-unique indexes or range
predicate on unique indexes (<.>, between etc)
Index skip scan Skips the leading edge (column) of the index & uses the rest Advantageous if there are few distinct values in the
leading column and many distinct values in the non-leading column or columns of the index
Full index scan Processes all leaf blocks of an index, but only enough branch blocks to find 1st leaf block. Used when all necessary
columns are in index & order by clause matches index structure or if a sort merge join is done
Fast full index scan Scans all blocks in index used to replace a Full Table Scan when all necessary columns are in the index. Using
multi-block IO & can going parallel
Index joins Hash join of several indexes that together contain all the table columns that are referenced in the query. Won’t
eliminate a sort operation
Bitmap indexes Uses a bitmap for key values and a mapping function that converts each bit position to a rowid. Can efficiently
merge indexes that correspond to several conditions in a WHERE clause
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Identifying access paths in an execution plan
If the wrong access method is being used check cardinality, join order…
Look in Operation section to see how an
object is being accessed
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Access path example 1
Table countries contains 10K rows & has a primary key on country_id
SELECT country_id, name
FROM countries
WHERE country_id IN ('AU','FR','IE‘);
What plan would you expect for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Access path example 2
Table countries contains 10K rows & has a primary key on country_id
SELECT country_id, name
FROM countries
WHERE country_id BETWEEN 'AU' AND 'IE';
What plan would you expect for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Access path example 3
Table countries contains 10K rows & has a primary key on country_id
SELECT country_id, name
FROM countries
WHERE name = ‘USA';
What plan would you expect for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Common access path issues
Issue Cause
Uses a table scan instead of index DOP on table but not index, value of MBRC
Picks wrong index Stale or missing statistics
Cost of full index access is cheaper than index look
up followed by table access
Picks index that matches most # of column
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Agenda
What is an execution plan
How to generate a plan
What is a good plan for the optimizer
Understanding execution plans
• Cardinality
• Access paths
• Join methods
• Join order
Execution plan examples
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join methods
Join Methods Explanation
Nested Loops joins For every row in the outer table, Oracle accesses all the rows in the inner table Useful
when joining small subsets of data and there is an efficient way to access the
second table (index look up)
Hash Joins The smaller of two tables is scan and resulting rows are used to build a hash table on
the join key in memory. The larger table is then scan, join column of the resulting
rows are hashed and the values used to probing the hash table to find the matching
rows. Useful for larger tables & if equality predicate
Sort Merge joins Consists of two steps:
1. Sort join operation: Both the inputs are sorted on the join key.
2. Merge join operation: The sorted lists are merged together.
Useful when the join condition between two tables is an inequality condition
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join types
Join Type Explanation
Inner Joins Returns all rows that satisfy the join condition
Outer Joins Returns all rows that satisfy the join condition and also returns all of the rows from the table
without the (+) for which no rows from the other table satisfy the join condition
Cartesian
Joins
Joins every row from one data source with every row from the other data source, creating
the Cartesian Product of the two sets. Only good if tables are very small. Only choice if
there is no join condition specified in query
Semi-Join Returns a row from the outer table when a matching row exists in the subquery data set.
Typically used when there is an EXISTS or an IN predicate, where we aren’t interested in
returning rows from the subquery but merely checking a match exists
Anti-Join Returns a row from the outer table when a matching row does not exist in the subquery data
set. Typically used when there is a NOT EXISTS or NOT IN predicate, where we aren’t
interested in returning rows from the subquery but merely checking a match doesn’t exists
A B
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Identifying join methods in an execution plan
If wrong join type is used check stmt is written correctly & cardinality estimates
Look in the Operation section to check
the right join type is used
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join method example 1
SELECT e.last_name, e.salary, d.department_name
FROM hr.employees e, hr.departments d
WHERE d.departments_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
What join method would you expect for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join method example 1
SELECT e.last_name, e.salary, d.department_name
FROM hr.employees e, hr.departments d
WHERE d.departments_name IN ('Marketing‘,'Sales')
AND e.department_id = d.department_id;
What join method would you expect for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join method 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
What join method would you expect for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join method 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
What join method would you expect for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join type example 1
SELECT o.order_id, o.order_date ,e.name
FROM oe.orders o , hr.employees e;
Orders has 105 rows
Employees has 107 rows
What join type should be use for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join type example 1
SELECT o.order_id, o.order_date, e.name
FROM oe.orders o , hr.employees e;
What join type should be use for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join type example 2
• Chosen when the number of rows being joined is low
• Commonly used to join multiple small dimensions to one large fact table
Cartesian product not always bad
s.prod_id=p.prod_id s.cust_id=c.cust_id
s.promo_id=c.promo_id
CUSTOMERSPRODUCTS
PROMOTIONS
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join type example 2
• By joining the three small dimension tables together first we minimize the
number of rows being carried through the joins
Cartesian product not always bad
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join type example 3
SELECT d.department_id,e.emp_id
FROM hr.employees e FULL OUTER JOIN hr.departments d
ON e.department_id = d.department_id
ORDER BY d.department_id;
Employees has 107 rows
Departments has 27 rows
Foreign key relationship between Employees and Departments on dept_id
What join type should be use for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join type example 3
SELECT d.department_id,e.emp_id
FROM hr.employees e FULL OUTER JOIN hr.departments d
ON e.department_id = d.department_id
ORDER BY d.department_id;
What join type should be use for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join type example 4
SELECT s.quantity_sold
FROM sales s, customers c
WHERE s.cust_id =c.cust_id;
Sales table has 960 Rows
Customer table has 55,500 rows
Customer has a primary key created on cust_id
Sales has a foreign key created on cust_id
What join type should be use for this query?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What join type should be use for this query?
Join type example 4
SELECT s.quantity_sold
FROM sales s, customers c
WHERE s.cust_id =c.cust_id;
No join is needed
Table elimination transformation
Optimizer realizes that the join to
customers tables is redundant as no
columns are selected Presence of
primary –foreign key relationship
means we can remove table
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What causes wrong join method to be selected
Issue Cause
Nested loop selected instead of hash join Cardinality estimate on the left side is
under estimated triggers Nested loop to
be selected
Hash join selected instead of nested loop In case of a hash join the Optimizer
doesn’t taken into consideration the
benefit of caching. If rows on the left
come in a clustered or ordered fashion
so the probe into 2nd table is more
efficient
Cartesian Joins Cardinality underestimation
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Agenda
What is an execution plan
How to generate a plan
What is a good plan for the optimizer
Understanding execution plans
• Cardinality
• Access paths
• Join methods
• Join order
Execution plan examples
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Join order
The order in which the tables are join in a multi table statement
• Ideally start with the table that will eliminate the most rows
• Strongly affected by the access paths available
Some basic rules
• Joins guaranteed to produce at most one row always go first
– Joins between two row sources that have only one row each
• When outer joins are used the table with the outer join operator must come
after the other table in the predicate
• If view merging is not possible all tables in the view will be joined before
joining to the tables outside the view
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Identifying join order in an execution plan
If the join order is not correct, check the statistics, cardinality & access methods
1
2
3
Want to start with the table that
reduce the result set the most
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
• It can be hard to determine Join Order for Complex SQL statements but it is
easily visible in the outline data of plan FORMAT=>’TYPICAL +outline’);
SELECT * FROM table(dbms_xplan.display_cursor(format=>TYPICAL +OUTLINE'));
Finding the join order for complex SQL
The leading hint tells
you the join order
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What causes the wrong join order
Causes
Incorrect single table cardinality estimates
Incorrect join cardinality estimates
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Agenda
What is an execution plan
How to generate a plan
What is a good plan for the optimizer
Understanding execution plans
Execution plan examples
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Example SQL Statement
SELECT e1.last_name, e1.job_title, e1.total_comp
FROM (SELECT e.manager_id, e.last_name, j.job_title,
e.salary+(e.salary+e.commission_pct) total_comp
FROM employees e, jobs j, departments d
WHERE d.department_name = 'Sales'
AND e.department_id = d.department_id
AND e.job_id = j.job_id ) e1,
(SELECT e.employee_id, e.salary+(e.salary+e.commission_pct) tc
FROM employees e, departments d
WHERE d.department_name = ‘Sales'
AND e.department_id = d.department_id ) e2
WHERE e1.manager_id = e2.employee_id
AND e1.total_comp >= e2.tc;
Find all the employees who make as much or more than their manager
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Is it a good execution plan?
Means no stats gathered strong
indicator this won’t be best
possible plan
1. Is the estimated number of rows
being returned accurate?
2. Are the cardinality
estimates accurate?
3.Are the access
method correct?
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Example cont’d execution plan
5. Is the join order correct? Is the table that
eliminates the most rows accessed first?
4. Are the right join methods being used?
1
2
3
4
5
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What does the plan tree look like?
TABLE ACCESS
EMPLOYEES
TABLE ACCESS
DEPARTMENT
MERGE JOIN CARTESIAN TABLE ACCESS
EMPLOYEES
HASH JOIN INDEX UNIQUE SCAN –
TABLE ACCESS DEPARTMENT
NESTED LOOP INDEX UNIQUE SCAN –
TABLE ACCESS JOBS
NESTED LOOP
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
Solution
2. Cardinalities are correct and with
each join number of rows reduced
1. Only 1 row is actually returned and the cost is 4
lower now
4. Join methods have
changed to be all NL
3. Access methods have
changed for some
tables
1
2
3
5. The join
order has
changed
5
4
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. |
What does the plan tree look like?
TABLE ACCESS
DEPARTMENT
NESTED LOOP
INDEX UNIQUE SCAN –
TABLE ACCESS DEPARTMENT
NESTED LOOP
INDEX UNIQUE SCAN -
TABLE ACCESS JOBS
NESTED LOOP
NESTED LOOP
INDEX RANGE SCAN –
TABLE ACCESS EMPLOYEES
INDEX RANGE SCAN -
TABLE ACCESS EMPLOYEES
Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
If you have more questions later, feel free to ask
Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | 64
Related White Papers
• Explain the Explain Plan
• Understanding Optimizer Statistics
• Best Practices for Gathering Optimizer Statistics
• What to expect from the Optimizer in 12c
• What to expect from the Optimizer in 11g
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Explain the explain_plan

  • 1. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. | Explaining the Explain Plan Maria Colgan Master Product Manager Oracle Database Server Technologies June 2018 Interpreting Execution Plans for SQL Statements
  • 2. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Safe Harbor Statement The following is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle’s products remains at the sole discretion of Oracle. 2
  • 3. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Agenda What is an execution plan How to generate a plan What is a good plan for the Optimizer Understanding execution plans Execution plan examples 1 2 3 4 5
  • 4. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What is an execution plan? • Execution plans show the detailed steps necessary to execute a SQL statement • These steps are expressed as a set of database operators that consumes and produces rows • The order of the operators and their implementation is decided by the optimizer using a combination of query transformations and physical optimization techniques • The display is commonly shown in a tabular format, but a plan is in fact tree- shaped
  • 5. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What is an execution plan? Query: SELECT prod_category, avg(amount_sold) FROM sales s, products p WHERE p.prod_id = s.prod_id GROUP BY prod_category; HASH JOIN TABLE ACCESS SALES Tree-shaped representation of plan TABLE ACCESS PRODUCTS GROUP BY Tabular representation of plan
  • 6. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Agenda What is an execution plan How to generate a plan What is a good plan for the Optimizer Understanding execution plans Execution plan examples 1 2 3 4 5
  • 7. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | • Autotrace • SQL Monitor 7 • SQL Developer • TKPROF Many Ways to View an Execution Plan …..But there are actually only 2 ways to generate one
  • 8. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | How to generate an execution plan 1.EXPLAIN PLAN command – Displays an execution plan for a SQL statement without actually executing the statement 2.V$SQL_PLAN – A dictionary view introduced in Oracle 9i that shows the execution plan for a SQL statement that has been compiled into a cursor in the cursor cache Two methods for looking at the execution plan Under certain conditions the plan shown with EXPLAIN PLAN can be different from the plan shown using V$SQL_PLAN
  • 9. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | How to generate an execution plan SQL> EXPLAIN PLAN FOR SELECT p.prod_name, avg(s.amount_sold) FROM sales s, products p WHERE p.prod_id = s.prod_id GROUP BY p.prod_name; SQL> SELECT * FROM table(dbms_xplan.display('plan_table',null,'basic')); EXPLAIN PLAN command & dbms_xplan.display function PLAN TABLE NAME STATEMENT ID FORMAT
  • 10. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | How to generate an execution plan SQL> SELECT p.prod_name, avg(s.amount_sold) FROM sales s, products p WHERE p.prod_id = s.prod_id GROUP BY p.prod_name; SQL> SELECT * FROM table(dbms_xplan.display_cursor(null, null, 'basic')); Generate & display execution plan for last SQL statements executed in session SQL_ID CHILD NUMBER FORMAT • Format* is highly customizable - Basic ,Typical, All – Additional low level parameters show more detail *More information on formatting on Optimizer blog
  • 11. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Agenda What is an execution plan How to generate a plan What is a good plan for the Optimizer Understanding execution plans Execution plan examples 1 2 3 4 5
  • 12. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What’s a good plan for the Optimizer? The Optimizer has two different goals • Serial execution: It’s all about cost – The cheaper, the better • Parallel execution: it’s all about performance – The faster, the better Two fundamental questions: • What is cost • What is performance
  • 13. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What is cost? • A magically number the optimizer makes up? • Resources required to execute a SQL statement? • Estimate of how long it will take to execute a statement? Actual Definition • Cost represents units of work or resources used • Optimizer uses CPU & IO as units of work • Estimate of amount of CPU & disk I/Os, used to perform an operation Cost is an internal Oracle measurement & should be used for comparison purposes only
  • 14. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What is performance? • Getting as many queries completed as possible? • Getting fastest possible elapsed time using the fewest resources? • Getting the best concurrency rate? Actual Definition • Performance is fastest possible response time for a query • Goal is to complete the query as quickly as possible • Optimizer does not focus on resources needed to execute the plan
  • 15. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Agenda What is an execution plan How to generate a plan What is a good plan for the optimizer Understanding execution plans • Cardinality • Access paths • Join methods • Join order Execution plan examples 1 2 3 4 5
  • 16. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Cardinality What is it? • Estimate of number rows that will be returned by each operation How does the Optimizer Determine it? • Cardinality for a single column equality predicate = total num of rows num of distinct values – For example: A table has 100 rows, a column has 5 distinct values => cardinality=20 rows • More complicated predicates have more complicated cardinality calculation Why should you care? It influences everything! Access method, Join type, Join Order etc
  • 17. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Identifying cardinality in an execution plan Cardinality - estimated # of rows returned Determine correct cardinality using a SELECT COUNT(*) from each table applying any WHERE Clause predicates belonging to that table
  • 18. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Checking cardinality estimates SELECT /*+ gather_plan_statistics */ p.prod_name, SUM(s.quantity_sold) FROM sales s, products p WHERE s.prod_id =p.prod_id GROUP BY p.prod_name ; SELECT * FROM table ( DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST'));
  • 19. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Checking cardinality estimates SELECT * FROM table ( DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST')); Compare estimated number of rows (E-Rows) with actual rows returned (A-Rows)
  • 20. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Checking cardinality estimates SELECT * FROM table ( DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST')); Extra information you get with ALLSTATS Starts indicates the number of times that step or operation was done In this case the SALES table is partitioned and has 28 partitions
  • 21. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Checking cardinality estimates SELECT * FROM table ( DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST')); Extra information you get with ALLSTATS Buffers indicates the number of buffers that need to be read for each step
  • 22. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Checking cardinality estimates SELECT * FROM table ( DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST')); Extra information you get with ALLSTATS 0Mem - estimated amount of memory needed 1Mem - amount of memory need to perform the operation in 1 pass Used-Mem - actual amount of memory used and number of passes required
  • 23. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Checking cardinality estimates for Parallel Execution SELECT * FROM table ( 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
  • 24. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Checking cardinality estimates for Parallel Execution SELECT * FROM table ( DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS ALL'));
  • 25. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Easiest way to compare the estimated number of rows returned with actual rows returned Check cardinality using SQL Monitor
  • 26. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Solutions to incorrect cardinality estimates Cause Solution Stale or missing statistics DBMS_STATS Data Skew Create a histogram Multiple single column predicates on a table Create a column group using DBMS_STATS.CREATE_EXTENDED_STATS Function wrapped column Create statistics on the funct wrapped column using DBMS_STATS.CREATE_EXTENDED_STATS Multiple columns used in a join Create a column group on join columns using DBMS_STATS.CREATE_EXTENDED_STAT Complicated expression containing columns from multiple tables Use dynamic sampling level 4 or higher
  • 27. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Agenda What is an execution plan How to generate a plan What is a good plan for the optimizer Understanding execution plans • Cardinality • Access paths • Join methods • Join order Execution plan examples 1 2 3 4 5
  • 28. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Access paths – Getting the data Access Path Explanation Full table scan Reads all rows from table & filters out those that do not meet the where clause predicates. Used when no index, DOP set etc Table access by Rowid Rowid specifies the datafile & data block containing the row and the location of the row in that block. Used if rowid supplied by index or in where clause Index unique scan Only one row will be returned. Used when table contains a UNIQUE or a PRIMARY KEY constraint that guarantees that only a single row is accessed Index range scan Accesses adjacent index entries returns ROWID values Used with equality on non-unique indexes or range predicate on unique indexes (<.>, between etc) Index skip scan Skips the leading edge (column) of the index & uses the rest Advantageous if there are few distinct values in the leading column and many distinct values in the non-leading column or columns of the index Full index scan Processes all leaf blocks of an index, but only enough branch blocks to find 1st leaf block. Used when all necessary columns are in index & order by clause matches index structure or if a sort merge join is done Fast full index scan Scans all blocks in index used to replace a Full Table Scan when all necessary columns are in the index. Using multi-block IO & can going parallel Index joins Hash join of several indexes that together contain all the table columns that are referenced in the query. Won’t eliminate a sort operation Bitmap indexes Uses a bitmap for key values and a mapping function that converts each bit position to a rowid. Can efficiently merge indexes that correspond to several conditions in a WHERE clause
  • 29. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Identifying access paths in an execution plan If the wrong access method is being used check cardinality, join order… Look in Operation section to see how an object is being accessed
  • 30. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Access path example 1 Table countries contains 10K rows & has a primary key on country_id SELECT country_id, name FROM countries WHERE country_id IN ('AU','FR','IE‘); What plan would you expect for this query?
  • 31. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Access path example 2 Table countries contains 10K rows & has a primary key on country_id SELECT country_id, name FROM countries WHERE country_id BETWEEN 'AU' AND 'IE'; What plan would you expect for this query?
  • 32. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Access path example 3 Table countries contains 10K rows & has a primary key on country_id SELECT country_id, name FROM countries WHERE name = ‘USA'; What plan would you expect for this query?
  • 33. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Common access path issues Issue Cause Uses a table scan instead of index DOP on table but not index, value of MBRC Picks wrong index Stale or missing statistics Cost of full index access is cheaper than index look up followed by table access Picks index that matches most # of column
  • 34. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Agenda What is an execution plan How to generate a plan What is a good plan for the optimizer Understanding execution plans • Cardinality • Access paths • Join methods • Join order Execution plan examples 1 2 3 4 5
  • 35. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join methods Join Methods Explanation Nested Loops joins For every row in the outer table, Oracle accesses all the rows in the inner table Useful when joining small subsets of data and there is an efficient way to access the second table (index look up) Hash Joins The smaller of two tables is scan and resulting rows are used to build a hash table on the join key in memory. The larger table is then scan, join column of the resulting rows are hashed and the values used to probing the hash table to find the matching rows. Useful for larger tables & if equality predicate Sort Merge joins Consists of two steps: 1. Sort join operation: Both the inputs are sorted on the join key. 2. Merge join operation: The sorted lists are merged together. Useful when the join condition between two tables is an inequality condition
  • 36. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join types Join Type Explanation Inner Joins Returns all rows that satisfy the join condition Outer Joins Returns all rows that satisfy the join condition and also returns all of the rows from the table without the (+) for which no rows from the other table satisfy the join condition Cartesian Joins Joins every row from one data source with every row from the other data source, creating the Cartesian Product of the two sets. Only good if tables are very small. Only choice if there is no join condition specified in query Semi-Join Returns a row from the outer table when a matching row exists in the subquery data set. Typically used when there is an EXISTS or an IN predicate, where we aren’t interested in returning rows from the subquery but merely checking a match exists Anti-Join Returns a row from the outer table when a matching row does not exist in the subquery data set. Typically used when there is a NOT EXISTS or NOT IN predicate, where we aren’t interested in returning rows from the subquery but merely checking a match doesn’t exists A B
  • 37. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Identifying join methods in an execution plan If wrong join type is used check stmt is written correctly & cardinality estimates Look in the Operation section to check the right join type is used
  • 38. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join method example 1 SELECT e.last_name, e.salary, d.department_name FROM hr.employees e, hr.departments d WHERE d.departments_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 What join method would you expect for this query?
  • 39. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join method example 1 SELECT e.last_name, e.salary, d.department_name FROM hr.employees e, hr.departments d WHERE d.departments_name IN ('Marketing‘,'Sales') AND e.department_id = d.department_id; What join method would you expect for this query?
  • 40. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join method 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 What join method would you expect for this query?
  • 41. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join method 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 What join method would you expect for this query?
  • 42. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join type example 1 SELECT o.order_id, o.order_date ,e.name FROM oe.orders o , hr.employees e; Orders has 105 rows Employees has 107 rows What join type should be use for this query?
  • 43. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join type example 1 SELECT o.order_id, o.order_date, e.name FROM oe.orders o , hr.employees e; What join type should be use for this query?
  • 44. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join type example 2 • Chosen when the number of rows being joined is low • Commonly used to join multiple small dimensions to one large fact table Cartesian product not always bad s.prod_id=p.prod_id s.cust_id=c.cust_id s.promo_id=c.promo_id CUSTOMERSPRODUCTS PROMOTIONS
  • 45. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join type example 2 • By joining the three small dimension tables together first we minimize the number of rows being carried through the joins Cartesian product not always bad
  • 46. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join type example 3 SELECT d.department_id,e.emp_id FROM hr.employees e FULL OUTER JOIN hr.departments d ON e.department_id = d.department_id ORDER BY d.department_id; Employees has 107 rows Departments has 27 rows Foreign key relationship between Employees and Departments on dept_id What join type should be use for this query?
  • 47. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join type example 3 SELECT d.department_id,e.emp_id FROM hr.employees e FULL OUTER JOIN hr.departments d ON e.department_id = d.department_id ORDER BY d.department_id; What join type should be use for this query?
  • 48. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join type example 4 SELECT s.quantity_sold FROM sales s, customers c WHERE s.cust_id =c.cust_id; Sales table has 960 Rows Customer table has 55,500 rows Customer has a primary key created on cust_id Sales has a foreign key created on cust_id What join type should be use for this query?
  • 49. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What join type should be use for this query? Join type example 4 SELECT s.quantity_sold FROM sales s, customers c WHERE s.cust_id =c.cust_id; No join is needed Table elimination transformation Optimizer realizes that the join to customers tables is redundant as no columns are selected Presence of primary –foreign key relationship means we can remove table
  • 50. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What causes wrong join method to be selected Issue Cause Nested loop selected instead of hash join Cardinality estimate on the left side is under estimated triggers Nested loop to be selected Hash join selected instead of nested loop In case of a hash join the Optimizer doesn’t taken into consideration the benefit of caching. If rows on the left come in a clustered or ordered fashion so the probe into 2nd table is more efficient Cartesian Joins Cardinality underestimation
  • 51. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Agenda What is an execution plan How to generate a plan What is a good plan for the optimizer Understanding execution plans • Cardinality • Access paths • Join methods • Join order Execution plan examples 1 2 3 4 5
  • 52. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Join order The order in which the tables are join in a multi table statement • Ideally start with the table that will eliminate the most rows • Strongly affected by the access paths available Some basic rules • Joins guaranteed to produce at most one row always go first – Joins between two row sources that have only one row each • When outer joins are used the table with the outer join operator must come after the other table in the predicate • If view merging is not possible all tables in the view will be joined before joining to the tables outside the view
  • 53. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Identifying join order in an execution plan If the join order is not correct, check the statistics, cardinality & access methods 1 2 3 Want to start with the table that reduce the result set the most 4 5
  • 54. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | • It can be hard to determine Join Order for Complex SQL statements but it is easily visible in the outline data of plan FORMAT=>’TYPICAL +outline’); SELECT * FROM table(dbms_xplan.display_cursor(format=>TYPICAL +OUTLINE')); Finding the join order for complex SQL The leading hint tells you the join order
  • 55. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What causes the wrong join order Causes Incorrect single table cardinality estimates Incorrect join cardinality estimates
  • 56. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Agenda What is an execution plan How to generate a plan What is a good plan for the optimizer Understanding execution plans Execution plan examples 1 2 3 4 5
  • 57. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Example SQL Statement SELECT e1.last_name, e1.job_title, e1.total_comp FROM (SELECT e.manager_id, e.last_name, j.job_title, e.salary+(e.salary+e.commission_pct) total_comp FROM employees e, jobs j, departments d WHERE d.department_name = 'Sales' AND e.department_id = d.department_id AND e.job_id = j.job_id ) e1, (SELECT e.employee_id, e.salary+(e.salary+e.commission_pct) tc FROM employees e, departments d WHERE d.department_name = ‘Sales' AND e.department_id = d.department_id ) e2 WHERE e1.manager_id = e2.employee_id AND e1.total_comp >= e2.tc; Find all the employees who make as much or more than their manager
  • 58. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Is it a good execution plan? Means no stats gathered strong indicator this won’t be best possible plan 1. Is the estimated number of rows being returned accurate? 2. Are the cardinality estimates accurate? 3.Are the access method correct?
  • 59. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Example cont’d execution plan 5. Is the join order correct? Is the table that eliminates the most rows accessed first? 4. Are the right join methods being used? 1 2 3 4 5
  • 60. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What does the plan tree look like? TABLE ACCESS EMPLOYEES TABLE ACCESS DEPARTMENT MERGE JOIN CARTESIAN TABLE ACCESS EMPLOYEES HASH JOIN INDEX UNIQUE SCAN – TABLE ACCESS DEPARTMENT NESTED LOOP INDEX UNIQUE SCAN – TABLE ACCESS JOBS NESTED LOOP
  • 61. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | Solution 2. Cardinalities are correct and with each join number of rows reduced 1. Only 1 row is actually returned and the cost is 4 lower now 4. Join methods have changed to be all NL 3. Access methods have changed for some tables 1 2 3 5. The join order has changed 5 4
  • 62. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | What does the plan tree look like? TABLE ACCESS DEPARTMENT NESTED LOOP INDEX UNIQUE SCAN – TABLE ACCESS DEPARTMENT NESTED LOOP INDEX UNIQUE SCAN - TABLE ACCESS JOBS NESTED LOOP NESTED LOOP INDEX RANGE SCAN – TABLE ACCESS EMPLOYEES INDEX RANGE SCAN - TABLE ACCESS EMPLOYEES
  • 63. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |Copyright © 2017, Oracle and/or its affiliates. All rights reserved. | If you have more questions later, feel free to ask
  • 64. Copyright © 2016, Oracle and/or its affiliates. All rights reserved. | 64 Related White Papers • Explain the Explain Plan • Understanding Optimizer Statistics • Best Practices for Gathering Optimizer Statistics • What to expect from the Optimizer in 12c • What to expect from the Optimizer in 11g Join the Conversation https://twitter.com/SQLMaria https://blogs.oracle.com/optimizer/ https://sqlmaria.com https://www.facebook.com/SQLMaria