Explain plan and TKPROF to tune databases and applications

Use EXPLAIN PLAN and TKPROF to Tune Your Applications
Summary:
 EXPLAIN PLAN overview
 TKPROF overview
 Why???
 Reading execution plans
 Reading TKPROF reports

Execution Plans and EXPLAIN PLAN
 An execution plan is a list of steps that Oracle will follow in order to execute
a SQL statement. Each step is one of a finite number of basic operations
known to the database server. Even the most complex SQL statement can
be broken down into a series of basic operations.
 EXPLAIN PLAN is a statement that allows you to have Oracle generate the
execution plan for any SQL statement without actually executing it. You will
be able to examine the execution plan by querying the plan table.
The Plan Table
 A plan table holds execution plans generated by the EXPLAIN PLAN
statement.
 The typical name for a plan table is plan_table, but you may use any name
you wish.
 Create the plan table by running utlxplan.sql, located in
$ORACLE_HOME/rdbms/admin.
Important Columns in the Plan Table
statement_id Unique identifier for each execution plan
timestamp When the execution plan was generated
operation The operation performed in one step of the execution plan, such as
“table access”
options Additional information about the operation, such as “by index
ROWID”
object_name Name of table, index, view, etc. accessed
optimizer Optimizer goal used when creating execution plan
id Step number in execution plan
parent_id Step number of parent step
EXPLAIN PLAN Prerequisites
 INSERT privilege on a plan table
 All necessary privileges to execute the statement being explained
 SELECT privileges on underlying tables of views, if the statement being
explained involves views
EXPLAIN PLAN Syntax
EXPLAIN PLAN
[SET STATEMENT_ID = ]
[INTO ]
FOR
Use EXPLAIN PLAN and TKPROF To Tune Your Applications i
;
Querying an Execution Plan from the Plan Table
 Use a CONNECT BY clause to trace the hierarchy
 Use LPAD function to indent rows, making the hierarchy easier to follow
 Put statement_id in WHERE clause to retrieve only one execution plan at a
time
 Sample script on next slide shows the most important information
 You can also try utlxpls.sql or utlxplp.sql in $ORACLE_HOME/rdbms/admin
A Simple Query to Display Execution Plans
SET VERIFY OFF
ACCEPT stmt_id CHAR PROMPT "Enter statement_id: "
COL id FORMAT 999
COL parent_id FORMAT 999 HEADING "PARENT"
COL operation FORMAT a35 TRUNCATE
COL object_name FORMAT a30
SELECT id, parent_id, LPAD (' ', LEVEL - 1) ||
operation || ' ' || options operation,
object_name
FROM plan_table
WHERE statement_id = '&stmt_id'
START WITH id = 0
AND statement_id = '&stmt_id'
CONNECT BY PRIOR id = parent_id
AND statement_id = '&stmt_id';
A Sample Execution Plan
SQL> EXPLAIN PLAN SET statement_id = 'demo' FOR
2 SELECT a.customer_name, a.customer_number, b.invoice_number,
3 b.invoice_type, b.invoice_date, b.total_amount,
4 c.line_number, c.part_number, c.quantity, c.unit_cost
5 FROM customers a, invoices b, invoice_items c
6 WHERE c.invoice_id = :b1
7 AND c.line_number = :b2
8 AND b.invoice_id = c.invoice_id
9 AND a.customer_id = b.customer_id;
Explained.
SQL> @explain.sql
Enter statement_id: demo
ID PARENT OPERATION OBJECT_NAME
---- ------ ----------------------------------- -----------------
0 SELECT STATEMENT
1 0 NESTED LOOPS
2 1 NESTED LOOPS
3 2 TABLE ACCESS BY INDEX ROWID INVOICE_ITEMS
4 3 INDEX UNIQUE SCAN INVOICE_ITEMS_PK
5 2 TABLE ACCESS BY INDEX ROWID INVOICES
6 5 INDEX UNIQUE SCAN INVOICES_PK
7 1 TABLE ACCESS BY INDEX ROWID CUSTOMERS
8 7 INDEX UNIQUE SCAN CUSTOMERS_PK
Other Ways to View Execution Plans
Use EXPLAIN PLAN and TKPROF To Tune Your Applications ii
 The autotrace feature in SQL*Plus
 Performance tuning tools
Sample Autotrace Output in SQL*Plus
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=4 Card=1 Bytes=39)
1 0 NESTED LOOPS (Cost=4 Card=1 Bytes=39)
2 1 NESTED LOOPS (Cost=3 Card=1 Bytes=27)
3 2 TABLE ACCESS (BY INDEX ROWID) OF 'INVOICE_ITEMS' (Cost
=2 Card=1 Bytes=15)
4 3 INDEX (UNIQUE SCAN) OF 'INVOICE_ITEMS_PK' (UNIQUE) (
Cost=1 Card=2)
5 2 TABLE ACCESS (BY INDEX ROWID) OF 'INVOICES' (Cost=1 Ca
rd=2 Bytes=24)
6 5 INDEX (UNIQUE SCAN) OF 'INVOICES_PK' (UNIQUE)
7 1 TABLE ACCESS (BY INDEX ROWID) OF 'CUSTOMERS' (Cost=1 Car
d=100 Bytes=1200)
8 7 INDEX (UNIQUE SCAN) OF 'CUSTOMERS_PK' (UNIQUE)
EXPLAIN PLAN Limitations
 The EXPLAIN PLAN statement provides a good faith estimate of the
execution plan that Oracle would use. The real plan that gets used may
differ from what EXPLAIN PLAN tells you for many reasons:
– Optimizer stats, cursor sharing, bind variable peeking, dynamic
instance parameters make plans less stable.
– EXPLAIN PLAN does not peek at bind variables.
– EXPLAIN PLAN does not check the library cache to see if the
statement has already been parsed.
 EXPLAIN PLAN does not work for some queries:
ORA-22905: cannot access rows from a non-nested table item
Viewing Actual Execution Plans
 The v$sql view shows statements in the library cache. Here you can find the
address, hash value, and child number for a statement of interest.
 The v$sql_plan view shows the actual execution plan for each statement,
given its address, hash value, and child number. The columns are similar to
the plan table.
 The v$sql_plan_statistics view shows actual statistics (rows, buffer gets,
elapsed time, etc.) for each operation of the execution plan.
 The v$sql_plan and v$sql_plan_statistics views are available starting in
Oracle 9i. v$sql_plan_statistics is not populated by default.
A Simple Query to Display Actual Execution Plans
SET VERIFY OFF
COL id FORMAT 999
COL parent_id FORMAT 999 HEADING "PARENT"
COL operation FORMAT a35 TRUNCATE
COL object_name FORMAT a30
Use EXPLAIN PLAN and TKPROF To Tune Your Applications iii
SELECT id, parent_id, LPAD (' ', LEVEL - 1) ||
operation || ' ' || options operation, object_name
FROM (
SELECT id, parent_id, operation, options, object_name
FROM v$sql_plan
WHERE address = '&address'
AND hash_value = &hash_value
AND child_number = &child_number
)
START WITH id = 0
CONNECT BY PRIOR id = parent_id;
Trace Files and TKPROF
 The Oracle server process managing a database session writes a verbose
trace file when SQL trace is enabled for the session.
 TKPROF is a utility provided by Oracle that formats SQL trace files into very
helpful and readable reports. TKPROF is installed automatically when the
database server software is installed. You invoke TKPROF from the operating
system command line; there is no graphical interface for TKPROF. Starting in
Oracle 9i TKPROF can read extended SQL trace files and report on wait
events statistics.
Enabling SQL Trace
 At the instance level:
sql_trace = true
timed_statistics = true (optional)
 In your own session:
ALTER SESSION SET sql_trace = TRUE;
ALTER SESSION SET timed_statistics = TRUE; (optional)
 In another session:
SYS.dbms_system.set_sql_trace_in_session
(, , TRUE)
Finding the Trace File
 Look in the user dump destination. On OFA compliant systems this will be
$ORACLE_BASE/admin/$ORACLE_SID/udump
 Check timestamps and file contents to see which trace file is yours
 If non-DBAs need access to trace files, add _trace_files_public = true to the
parameter file to avoid permissions problems on Unix platforms
 Use a dedicated server connection when tracing, if possible.
Formatting a Trace File with TKPROF
Invoke TKPROF from the operating system prompt like this:
tkprof

\
[explain=] \
[sys=n] [sort=]
TKPROF Command-line Arguments
tkprof

\
[explain=] \
[sys=n] [sort=]
Use EXPLAIN PLAN and TKPROF To Tune Your Applications iv
trace file The SQL trace file to be formatted
output file The formatted output to be written by TKPROF
explain= Database login to be used if you want the output to include execution
plans
sys=n Omit “recursive SQL” performed by the SYS user
sort= List traced SQL statement in the output file in a specific order
TKPROF Sample Output
SELECT a.customer_name, a.customer_number, b.invoice_number,
b.invoice_type, b.invoice_date, b.total_amount, c.line_number,
c.part_number, c.quantity, c.unit_cost
FROM customers a, invoices b, invoice_items c
WHERE c.invoice_id = :b1
AND c.line_number = :b2
AND b.invoice_id = c.invoice_id
AND a.customer_id = b.customer_id
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.05 0.02 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 8 8 0 1
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 4 0.05 0.02 8 8 0 1
Misses in library cache during parse: 1
Optimizer goal: CHOOSE
Parsing user id: 34 (RSCHRAG)
Why Look at Execution Plans and TKPROF Reports?
These tools are critical to the application tuning process, and tuning at the
application level is necessary for high performance systems. With EXPLAIN
PLAN, v$sql_plan, and TKPROF, you can:
 Proactively tune applications that are in development
 Reactively tune production systems that are experiencing performance
problems
 Estimate resource needs or feasibility of ad-hoc queries
 Quantify resource needs for specific applications
Reading Execution Plans
An execution plan is a hierarchical listing of steps. Each step is one of a few
basic data access operations known to the database server. The most complex
SQL statement can be broken down into a series of basic operations.
 “Read from the most indented step outward.”
This is not exactly correct!
 Instead, take this approach:
a) Start at the least indented step
b) Find the step or steps that provide direct input to the step noted in (a).
c) Evaluate each of the steps found in (b). This may involve recursively finding
steps that provide input and evaluating them.
Execution Plan Example #1
SELECT customer_id, customer_name
Use EXPLAIN PLAN and TKPROF To Tune Your Applications v
FROM customers
WHERE UPPER (customer_name) LIKE 'ACME%'
ORDER BY customer_name;
OPERATION OBJECT_NAME
------------------------------ --------------
SELECT STATEMENT
SORT ORDER BY
TABLE ACCESS FULL CUSTOMERS
Execution Plan Operations
TABLE ACCESS FULL
Perform a full table scan of the indicated table and retrieve all rows that meet
criteria from the WHERE clause. Input: no subordinate operations. Output: the
necessary columns from the rows meeting all criteria.
SORT ORDER BY
Sort the input rows for the purpose of satisfying an ORDER BY clause. Input: the
rows to be sorted. Output: the rows in sorted order.
Execution Plan Example #2
SELECT a.customer_name, b.invoice_number,
b.invoice_date
FROM customers a, invoices b
WHERE b.invoice_date > TRUNC (SYSDATE - 1)
AND a.customer_id = b.customer_id;
OPERATION OBJECT_NAME
------------------------------ --------------
SELECT STATEMENT
NESTED LOOPS
TABLE ACCESS BY INDEX ROWID INVOICES
INDEX RANGE SCAN INVOICES_DATE
TABLE ACCESS BY INDEX ROWID CUSTOMERS
INDEX UNIQUE SCAN CUSTOMERS_PK
Execution Plan Operations
INDEX UNIQUE SCAN
Look up a complete key in a unique index. Input: usually no subordinate
operations. (Key values typically come from the original query or a parent
operation.) Output: zero or one ROWIDs from the index.
INDEX RANGE SCAN
Look up a key in a non-unique index, or an incomplete key in a unique index.
Input: usually no subordinate operations. Output: zero or more ROWIDs from
the index.
Execution Plan Operations
TABLE ACCESS BY INDEX ROWID
Look up rows in a table by their ROWIDs. Input: a list of ROWIDs to look up.
Output: the necessary columns from the rows with the given ROWIDs.
NESTED LOOPS
Perform a join between two sets of row data using the nested loops algorithm.
Inputs: two separate sets of row data. Output: the results of the join.
Use EXPLAIN PLAN and TKPROF To Tune Your Applications vi
For each row Oracle reads from the first input, the operations that make up the
second input are executed once and matching rows generate output.
Execution Plan Example #3
SELECT a.customer_name,
COUNT (DISTINCT b.invoice_id) open_invs,
COUNT (c.invoice_id) open_inv_items
FROM customers a, invoices b, invoice_items c
WHERE b.invoice_status = 'OPEN'
AND a.customer_id = b.customer_id
AND c.invoice_id (+) = b.invoice_id
GROUP BY a.customer_name;
OPERATION OBJECT_NAME
-------------------------------- ----------------
SELECT STATEMENT
SORT GROUP BY
NESTED LOOPS OUTER
HASH JOIN
TABLE ACCESS BY INDEX ROWID INVOICES
INDEX RANGE SCAN INVOICES_STATUS
TABLE ACCESS FULL CUSTOMERS
INDEX RANGE SCAN INVOICE_ITEMS_PK
Execution Plan Operations
HASH JOIN
Perform a join between two sets of row data using the hash join algorithm.
Inputs: two separate sets of row data. Output: the results of the join.
Oracle reads all rows from the second input and builds a hash structure, before
reading each row from the first input one at a time. For each row from the first
input, the hash structure is probed and matching rows generate output.
Execution Plan Operations
NESTED LOOPS OUTER
Same as the NESTED LOOPS operation, except that an outer join is performed.
SORT GROUP BY
Same as the SORT ORDER BY operation, except that the rows are sorted and
grouped to satisfy a GROUP BY clause.
Execution Plan Example #4
SELECT customer_name
FROM customers a
WHERE EXISTS
(SELECT 1
FROM invoices_view b
WHERE b.customer_id = a.customer_id
AND number_of_lines > 100)
ORDER BY customer_name;
CREATE OR REPLACE VIEW invoices_view AS
SELECT a.invoice_id, a.customer_id,
COUNT(*) number_of_lines
FROM invoices a, invoice_items b
WHERE b.invoice_id = a.invoice_id
Use EXPLAIN PLAN and TKPROF To Tune Your Applications vii
GROUP BY a.invoice_id, a.customer_id;
OPERATION OBJECT_NAME
----------------------------------- -------------
SELECT STATEMENT
SORT ORDER BY
FILTER
TABLE ACCESS FULL CUSTOMERS
VIEW INVOICES_VIEW
FILTER
SORT GROUP BY
NESTED LOOPS
TABLE ACCESS BY INDEX ROWID INVOICES
INDEX RANGE SCAN INVS_CUST_ID
INDEX RANGE SCAN INV_ITEMS_PK
Execution Plan Operations
FILTER
Read a set of row data and discard some rows based on various criteria. To
determine the criteria, operations from a second input may need to be
performed. Input: rows to be examined and, sometimes, an additional
subordinate operation that must be performed for each row from the first input
in order to evaluate criteria. Output: the rows from the first input that met the
criteria.
VIEW
Build a physical representation of a database view or subset of a database
view. Input: set of row data. Output: set of row data that implements the view
or subset of the view.
Notes on Execution Plan Operations
 The optimizer rewrites subqueries as joins and merges them into the main
query whenever possible.
 If a subquery is completely independent of the main query and cannot be
merged into the main query, the optimizer may treat the subquery as a
separate statement and leave it out of the execution plan for the main
query.
 The optimizer expands view definitions and merges them into the main
query wherever possible. A VIEW operation will only appear in an execution
plan when the view definition could not be merged.
Execution Plan Example #5
SELECT /*+ RULE */ a.cust_name, b.contact_name
FROM customers a, contacts@sales.acme.com b
WHERE UPPER(b.contact_name) = UPPER(a.cust_name);
Execution Plan
------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: RULE
1 0 MERGE JOIN
2 1 SORT (JOIN)
3 2 REMOTE* SALES.ACME.COM
4 1 SORT (JOIN)
5 4 TABLE ACCESS (FULL) OF 'CUSTOMERS'
Use EXPLAIN PLAN and TKPROF To Tune Your Applications viii
3 SERIAL_FROM_REMOTE SELECT "CONTACT_NAME"
FROM "CONTACTS" "B”
Execution Plan Operations
REMOTE
Submit a SQL statement to a remote database via Oracle Net. Input: typically
no subordinate operations. Output: the results of the query from the remote
database. Note that the database link used to access the remote database and
the actual SQL submitted to the remote database will be accessible from the
execution plan.
SORT JOIN
Same as the SORT GROUP BY operation, except that the input is sorted by the
join column or columns in preparation for a join using the merge join algorithm.
MERGE JOIN
Perform a join between two sets of row data using the merge join algorithm.
Inputs: two separate sets of row data. Output: the results of the join.
Oracle reads rows from both inputs in an alternating fashion and merges
together matching rows in order to generate output. The two inputs are
assumed to be sorted on the join column or columns.
Summary of Operations
We have not covered all of the execution plan operations, but we have covered
some of the most common ones:
- TABLE ACCESS FULL
- TABLE ACCESS BY INDEX ROWID
- INDEX UNIQUE SCAN
- INDEX RANGE SCAN
- NESTED LOOPS
- NESTED LOOPS OUTER
- HASH JOIN
- MERGE JOIN
- FILTER
- VIEW
- REMOTE
- SORT ORDER BY
- SORT GROUP BY
- SORT JOIN
Elements of a TKPROF Report
 Report heading
– TKPROF version, date run, sort option, trace file
 One entry for each distinct SQL statement in trace file
– Listing of SQL statement
– OCI call statistics: count of parse, execute, and fetch calls, rows
processed, and time and I/O used
– Parse information: parsing user, recursive depth, library cache
misses, and optimizer mode
– Row source operation listing
– Execution plan listing (optional)
– Wait event listing (optional)
Use EXPLAIN PLAN and TKPROF To Tune Your Applications ix
 Report Summary
– OCI call statistics totals
– Counts of how many statements were found in the trace file, how
many were distinct, and how many were explained in the report.
Sample TKPROF Report Heading
TKPROF: Release 8.1.6.1.0 - Production on Wed Aug 9 19:06:36 2000
(c) Copyright 1999 Oracle Corporation. All rights reserved.
Trace file: example.trc
Sort options: default
************************************************************************
count = number of times OCI procedure was executed
cpu = cpu time in seconds executing
elapsed = elapsed time in seconds executing
disk = number of physical reads of buffers from disk
query = number of buffers gotten for consistent read
current = number of buffers gotten in current mode (usually for update)
rows = number of rows processed by the fetch or execute call
Sample OCI Call Statistics
SELECT table_name
FROM user_tables
ORDER BY table_name
call count cpu elapsed disk query current rows
------- ------ -------- ---------- --------- --------- --------- ---------
Parse 1 0.01 0.02 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 14 0.59 0.99 0 33633 0 194
------- ------ -------- ---------- --------- --------- --------- ---------
total 16 0.60 1.01 0 33633 0 194
Misses in library cache during parse: 1
Optimizer goal: CHOOSE
Parsing user id: RSCHRAG [recursive depth: 0]
What the TKPROF Sample on the Previous Slide Tells Us
 The application called on Oracle to parse this statement once while SQL
trace was enabled.
 The parse took 0.01 CPU seconds, 0.02 elapsed seconds.
 No disk I/Os or buffer gets took place during the parse, suggesting that no
misses in the dictionary cache.
 Oracle was called on to execute this statement once.
 The execution took under 0.01 CPU seconds.
 No disk I/Os or buffer gets took place during the execution. (Queries often
defer the work to the fetch phase.)
 Oracle was called on 14 times to perform a fetch, and a total of 194 rows
were returned.
 Fetching took 0.59 CPU seconds, 0.99 elapsed seconds.
Use EXPLAIN PLAN and TKPROF To Tune Your Applications x
 Fetching required 33,633 buffer gets in consistent mode, but no physical
reads were required.
 The statement was not in the library cache (shared pool) when the parse
call came in.
 The cost-based optimizer and a goal of “choose” were used to parse the
statement.
 The RSCHRAG user was connected to the database when the parse
occurred.
 This statement was executed directly by the application; it was not invoked
recursively by the SYS user or a database trigger.
Sample Row Source Operation Listing
Rows Row Source Operation
------- ---------------------------------------------------
194 SORT ORDER BY
194 NESTED LOOPS
195 NESTED LOOPS OUTER
195 NESTED LOOPS OUTER
195 NESTED LOOPS
11146 TABLE ACCESS BY INDEX ROWID OBJ$
11146 INDEX RANGE SCAN (object id 34)
11339 TABLE ACCESS CLUSTER TAB$
12665 INDEX UNIQUE SCAN (object id 3)
33 INDEX UNIQUE SCAN (object id 33)
193 TABLE ACCESS CLUSTER SEG$
387 INDEX UNIQUE SCAN (object id 9)
194 TABLE ACCESS CLUSTER TS$
388 INDEX UNIQUE SCAN (object id 7)
Sample Execution Plan Listing
Rows Execution Plan
------- ---------------------------------------------------
0 SELECT STATEMENT GOAL: CHOOSE
194 SORT (ORDER BY)
194 NESTED LOOPS
195 NESTED LOOPS (OUTER)
195 NESTED LOOPS (OUTER)
195 NESTED LOOPS
11146 TABLE ACCESS (BY INDEX ROWID) OF 'OBJ$'
11146 INDEX (RANGE SCAN) OF 'I_OBJ2' (UNIQUE)
11339 TABLE ACCESS (CLUSTER) OF 'TAB$'
12665 INDEX (UNIQUE SCAN) OF 'I_OBJ#' (NON-UNIQUE)
33 INDEX (UNIQUE SCAN) OF 'I_OBJ1' (UNIQUE)
193 TABLE ACCESS (CLUSTER) OF 'SEG$'
387 INDEX (UNIQUE SCAN) OF 'I_FILE#_BLOCK#' (NON-UNIQUE)
194 TABLE ACCESS (CLUSTER) OF 'TS$'
388 INDEX (UNIQUE SCAN) OF 'I_TS#' (NON-UNIQUE)
Notes About TKPROF Execution Plan Listings
 Execution plans are only included in TKPROF reports if the explain=
parameter is specified when TKPROF is invoked
 TKPROF will create and drop its own plan table if one does not already exist
Use EXPLAIN PLAN and TKPROF To Tune Your Applications xi
 The row counts on each step are actuals—not estimates. This can be very
helpful when troubleshooting queries that perform poorly.
 When TKPROF runs the EXPLAIN PLAN statement for a query, a different
execution plan could be returned than was actually used in the traced
session.
TKPROF Reports: More Than Just Execution Plans
 Listing of SQL statements and library cache miss information helps you
determine if applications are using Oracle’s shared SQL facility effectively.
 Parse, execute, and fetch call counts help you determine if applications are
using Oracle APIs effectively.
 CPU and I/O statistics help you zero in on resource-intensive SQL
statements.
 Row counts on individual steps of the execution plans help you rework
inefficient execution plans.
Wrapping Up
 Use EXPLAIN PLAN, queries against v$sql_plan, the autotrace facility in
SQL*Plus, or GUI tools to view execution plans.
 Use TKPROF to format SQL trace files for human readability.
 Execution plans and TKPROF reports give the DBA and application
developer a wealth of information that can be used to make applications
efficient and perform well.
 The catch: you need to know how to interpret execution plans and TKPROF
reports in order to get any benefit from them. You also ought to know when
to use EXPLAIN PLAN versus when to query v$sql_plan.
Resources
 Download this slide show, the accompanying white paper, and
many other useful presentations at:
www.dbspecialists.com/presentations
Contact Information
Roger Schrag
Database Specialists, Inc.
388 Market Street, Suite 400
San Francisco, CA 94111
Tel: 415/344-0500
Email: rschrag@dbspecialists.com
Web: www.dbspecialists.com
Use EXPLAIN PLAN and TKPROF To Tune Your Applications xii
Use EXPLAIN PLAN and TKPROF To Tune Your Applications xiiiUse EXPLAIN PLAN and TKPROF to Tune Your Applications
Summary:
 EXPLAIN PLAN overview
 TKPROF overview
 Why???
 Reading execution plans
 Reading TKPROF reports
www.dbspecialists.com/presentations
Execution Plans and EXPLAIN PLAN
 An execution plan is a list of steps that Oracle will follow in order to execute
a SQL statement. Each step is one of a finite number of basic operations
known to the database server. Even the most complex SQL statement can
be broken down into a series of basic operations.
 EXPLAIN PLAN is a statement that allows you to have Oracle generate the
execution plan for any SQL statement without actually executing it. You will
be able to examine the execution plan by querying the plan table.
The Plan Table
 A plan table holds execution plans generated by the EXPLAIN PLAN
statement.
 The typical name for a plan table is plan_table, but you may use any name
you wish.
 Create the plan table by running utlxplan.sql, located in
$ORACLE_HOME/rdbms/admin.
Important Columns in the Plan Table
statement_id Unique identifier for each execution plan
timestamp When the execution plan was generated
operation The operation performed in one step of the execution plan, such as
“table access”
options Additional information about the operation, such as “by index
ROWID”
object_name Name of table, index, view, etc. accessed
optimizer Optimizer goal used when creating execution plan
id Step number in execution plan
parent_id Step number of parent step
EXPLAIN PLAN Prerequisites
 INSERT privilege on a plan table
 All necessary privileges to execute the statement being explained
 SELECT privileges on underlying tables of views, if the statement being
explained involves views
EXPLAIN PLAN Syntax
EXPLAIN PLAN
[SET STATEMENT_ID = ]
[INTO ]
FOR
Use EXPLAIN PLAN and TKPROF To Tune Your Applications i
;
Querying an Execution Plan from the Plan Table
 Use a CONNECT BY clause to trace the hierarchy
 Use LPAD function to indent rows, making the hierarchy easier to follow
 Put statement_id in WHERE clause to retrieve only one execution plan at a
time
 Sample script on next slide shows the most important information
 You can also try utlxpls.sql or utlxplp.sql in $ORACLE_HOME/rdbms/admin
A Simple Query to Display Execution Plans
SET VERIFY OFF
ACCEPT stmt_id CHAR PROMPT "Enter statement_id: "
COL id FORMAT 999
COL parent_id FORMAT 999 HEADING "PARENT"
COL operation FORMAT a35 TRUNCATE
COL object_name FORMAT a30
SELECT id, parent_id, LPAD (' ', LEVEL - 1) ||
operation || ' ' || options operation,
object_name
FROM plan_table
WHERE statement_id = '&stmt_id'
START WITH id = 0
AND statement_id = '&stmt_id'
CONNECT BY PRIOR id = parent_id
AND statement_id = '&stmt_id';
A Sample Execution Plan
SQL> EXPLAIN PLAN SET statement_id = 'demo' FOR
2 SELECT a.customer_name, a.customer_number, b.invoice_number,
3 b.invoice_type, b.invoice_date, b.total_amount,
4 c.line_number, c.part_number, c.quantity, c.unit_cost
5 FROM customers a, invoices b, invoice_items c
6 WHERE c.invoice_id = :b1
7 AND c.line_number = :b2
8 AND b.invoice_id = c.invoice_id
9 AND a.customer_id = b.customer_id;
Explained.
SQL> @explain.sql
Enter statement_id: demo
ID PARENT OPERATION OBJECT_NAME
---- ------ ----------------------------------- -----------------
0 SELECT STATEMENT
1 0 NESTED LOOPS
2 1 NESTED LOOPS
3 2 TABLE ACCESS BY INDEX ROWID INVOICE_ITEMS
4 3 INDEX UNIQUE SCAN INVOICE_ITEMS_PK
5 2 TABLE ACCESS BY INDEX ROWID INVOICES
6 5 INDEX UNIQUE SCAN INVOICES_PK
7 1 TABLE ACCESS BY INDEX ROWID CUSTOMERS
8 7 INDEX UNIQUE SCAN CUSTOMERS_PK
Other Ways to View Execution Plans
Use EXPLAIN PLAN and TKPROF To Tune Your Applications ii
 The autotrace feature in SQL*Plus
 Performance tuning tools
Sample Autotrace Output in SQL*Plus
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=4 Card=1 Bytes=39)
1 0 NESTED LOOPS (Cost=4 Card=1 Bytes=39)
2 1 NESTED LOOPS (Cost=3 Card=1 Bytes=27)
3 2 TABLE ACCESS (BY INDEX ROWID) OF 'INVOICE_ITEMS' (Cost
=2 Card=1 Bytes=15)
4 3 INDEX (UNIQUE SCAN) OF 'INVOICE_ITEMS_PK' (UNIQUE) (
Cost=1 Card=2)
5 2 TABLE ACCESS (BY INDEX ROWID) OF 'INVOICES' (Cost=1 Ca
rd=2 Bytes=24)
6 5 INDEX (UNIQUE SCAN) OF 'INVOICES_PK' (UNIQUE)
7 1 TABLE ACCESS (BY INDEX ROWID) OF 'CUSTOMERS' (Cost=1 Car
d=100 Bytes=1200)
8 7 INDEX (UNIQUE SCAN) OF 'CUSTOMERS_PK' (UNIQUE)
EXPLAIN PLAN Limitations
 The EXPLAIN PLAN statement provides a good faith estimate of the
execution plan that Oracle would use. The real plan that gets used may
differ from what EXPLAIN PLAN tells you for many reasons:
– Optimizer stats, cursor sharing, bind variable peeking, dynamic
instance parameters make plans less stable.
– EXPLAIN PLAN does not peek at bind variables.
– EXPLAIN PLAN does not check the library cache to see if the
statement has already been parsed.
 EXPLAIN PLAN does not work for some queries:
ORA-22905: cannot access rows from a non-nested table item
Viewing Actual Execution Plans
 The v$sql view shows statements in the library cache. Here you can find the
address, hash value, and child number for a statement of interest.
 The v$sql_plan view shows the actual execution plan for each statement,
given its address, hash value, and child number. The columns are similar to
the plan table.
 The v$sql_plan_statistics view shows actual statistics (rows, buffer gets,
elapsed time, etc.) for each operation of the execution plan.
 The v$sql_plan and v$sql_plan_statistics views are available starting in
Oracle 9i. v$sql_plan_statistics is not populated by default.
A Simple Query to Display Actual Execution Plans
SET VERIFY OFF
COL id FORMAT 999
COL parent_id FORMAT 999 HEADING "PARENT"
COL operation FORMAT a35 TRUNCATE
COL object_name FORMAT a30
Use EXPLAIN PLAN and TKPROF To Tune Your Applications iii
SELECT id, parent_id, LPAD (' ', LEVEL - 1) ||
operation || ' ' || options operation, object_name
FROM (
SELECT id, parent_id, operation, options, object_name
FROM v$sql_plan
WHERE address = '&address'
AND hash_value = &hash_value
AND child_number = &child_number
)
START WITH id = 0
CONNECT BY PRIOR id = parent_id;
Trace Files and TKPROF
 The Oracle server process managing a database session writes a verbose
trace file when SQL trace is enabled for the session.
 TKPROF is a utility provided by Oracle that formats SQL trace files into very
helpful and readable reports. TKPROF is installed automatically when the
database server software is installed. You invoke TKPROF from the operating
system command line; there is no graphical interface for TKPROF. Starting in
Oracle 9i TKPROF can read extended SQL trace files and report on wait
events statistics.
Enabling SQL Trace
 At the instance level:
sql_trace = true
timed_statistics = true (optional)
 In your own session:
ALTER SESSION SET sql_trace = TRUE;
ALTER SESSION SET timed_statistics = TRUE; (optional)
 In another session:
SYS.dbms_system.set_sql_trace_in_session
(, , TRUE)
Finding the Trace File
 Look in the user dump destination. On OFA compliant systems this will be
$ORACLE_BASE/admin/$ORACLE_SID/udump
 Check timestamps and file contents to see which trace file is yours
 If non-DBAs need access to trace files, add _trace_files_public = true to the
parameter file to avoid permissions problems on Unix platforms
 Use a dedicated server connection when tracing, if possible.
Formatting a Trace File with TKPROF
Invoke TKPROF from the operating system prompt like this:
tkprof

\
[explain=] \
[sys=n] [sort=]
TKPROF Command-line Arguments
tkprof

\
[explain=] \
[sys=n] [sort=]
Use EXPLAIN PLAN and TKPROF To Tune Your Applications iv
trace file The SQL trace file to be formatted
output file The formatted output to be written by TKPROF
explain= Database login to be used if you want the output to include execution
plans
sys=n Omit “recursive SQL” performed by the SYS user
sort= List traced SQL statement in the output file in a specific order
TKPROF Sample Output
SELECT a.customer_name, a.customer_number, b.invoice_number,
b.invoice_type, b.invoice_date, b.total_amount, c.line_number,
c.part_number, c.quantity, c.unit_cost
FROM customers a, invoices b, invoice_items c
WHERE c.invoice_id = :b1
AND c.line_number = :b2
AND b.invoice_id = c.invoice_id
AND a.customer_id = b.customer_id
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.05 0.02 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 8 8 0 1
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 4 0.05 0.02 8 8 0 1
Misses in library cache during parse: 1
Optimizer goal: CHOOSE
Parsing user id: 34 (RSCHRAG)
Why Look at Execution Plans and TKPROF Reports?
These tools are critical to the application tuning process, and tuning at the
application level is necessary for high performance systems. With EXPLAIN
PLAN, v$sql_plan, and TKPROF, you can:
 Proactively tune applications that are in development
 Reactively tune production systems that are experiencing performance
problems
 Estimate resource needs or feasibility of ad-hoc queries
 Quantify resource needs for specific applications
Reading Execution Plans
An execution plan is a hierarchical listing of steps. Each step is one of a few
basic data access operations known to the database server. The most complex
SQL statement can be broken down into a series of basic operations.
 “Read from the most indented step outward.”
This is not exactly correct!
 Instead, take this approach:
a) Start at the least indented step
b) Find the step or steps that provide direct input to the step noted in (a).
c) Evaluate each of the steps found in (b). This may involve recursively finding
steps that provide input and evaluating them.
Execution Plan Example #1
SELECT customer_id, customer_name
Use EXPLAIN PLAN and TKPROF To Tune Your Applications v
FROM customers
WHERE UPPER (customer_name) LIKE 'ACME%'
ORDER BY customer_name;
OPERATION OBJECT_NAME
------------------------------ --------------
SELECT STATEMENT
SORT ORDER BY
TABLE ACCESS FULL CUSTOMERS
Execution Plan Operations
TABLE ACCESS FULL
Perform a full table scan of the indicated table and retrieve all rows that meet
criteria from the WHERE clause. Input: no subordinate operations. Output: the
necessary columns from the rows meeting all criteria.
SORT ORDER BY
Sort the input rows for the purpose of satisfying an ORDER BY clause. Input: the
rows to be sorted. Output: the rows in sorted order.
Execution Plan Example #2
SELECT a.customer_name, b.invoice_number,
b.invoice_date
FROM customers a, invoices b
WHERE b.invoice_date > TRUNC (SYSDATE - 1)
AND a.customer_id = b.customer_id;
OPERATION OBJECT_NAME
------------------------------ --------------
SELECT STATEMENT
NESTED LOOPS
TABLE ACCESS BY INDEX ROWID INVOICES
INDEX RANGE SCAN INVOICES_DATE
TABLE ACCESS BY INDEX ROWID CUSTOMERS
INDEX UNIQUE SCAN CUSTOMERS_PK
Execution Plan Operations
INDEX UNIQUE SCAN
Look up a complete key in a unique index. Input: usually no subordinate
operations. (Key values typically come from the original query or a parent
operation.) Output: zero or one ROWIDs from the index.
INDEX RANGE SCAN
Look up a key in a non-unique index, or an incomplete key in a unique index.
Input: usually no subordinate operations. Output: zero or more ROWIDs from
the index.
Execution Plan Operations
TABLE ACCESS BY INDEX ROWID
Look up rows in a table by their ROWIDs. Input: a list of ROWIDs to look up.
Output: the necessary columns from the rows with the given ROWIDs.
NESTED LOOPS
Perform a join between two sets of row data using the nested loops algorithm.
Inputs: two separate sets of row data. Output: the results of the join.
Use EXPLAIN PLAN and TKPROF To Tune Your Applications vi
For each row Oracle reads from the first input, the operations that make up the
second input are executed once and matching rows generate output.
Execution Plan Example #3
SELECT a.customer_name,
COUNT (DISTINCT b.invoice_id) open_invs,
COUNT (c.invoice_id) open_inv_items
FROM customers a, invoices b, invoice_items c
WHERE b.invoice_status = 'OPEN'
AND a.customer_id = b.customer_id
AND c.invoice_id (+) = b.invoice_id
GROUP BY a.customer_name;
OPERATION OBJECT_NAME
-------------------------------- ----------------
SELECT STATEMENT
SORT GROUP BY
NESTED LOOPS OUTER
HASH JOIN
TABLE ACCESS BY INDEX ROWID INVOICES
INDEX RANGE SCAN INVOICES_STATUS
TABLE ACCESS FULL CUSTOMERS
INDEX RANGE SCAN INVOICE_ITEMS_PK
Execution Plan Operations
HASH JOIN
Perform a join between two sets of row data using the hash join algorithm.
Inputs: two separate sets of row data. Output: the results of the join.
Oracle reads all rows from the second input and builds a hash structure, before
reading each row from the first input one at a time. For each row from the first
input, the hash structure is probed and matching rows generate output.
Execution Plan Operations
NESTED LOOPS OUTER
Same as the NESTED LOOPS operation, except that an outer join is performed.
SORT GROUP BY
Same as the SORT ORDER BY operation, except that the rows are sorted and
grouped to satisfy a GROUP BY clause.
Execution Plan Example #4
SELECT customer_name
FROM customers a
WHERE EXISTS
(SELECT 1
FROM invoices_view b
WHERE b.customer_id = a.customer_id
AND number_of_lines > 100)
ORDER BY customer_name;
CREATE OR REPLACE VIEW invoices_view AS
SELECT a.invoice_id, a.customer_id,
COUNT(*) number_of_lines
FROM invoices a, invoice_items b
WHERE b.invoice_id = a.invoice_id
Use EXPLAIN PLAN and TKPROF To Tune Your Applications vii
GROUP BY a.invoice_id, a.customer_id;
OPERATION OBJECT_NAME
----------------------------------- -------------
SELECT STATEMENT
SORT ORDER BY
FILTER
TABLE ACCESS FULL CUSTOMERS
VIEW INVOICES_VIEW
FILTER
SORT GROUP BY
NESTED LOOPS
TABLE ACCESS BY INDEX ROWID INVOICES
INDEX RANGE SCAN INVS_CUST_ID
INDEX RANGE SCAN INV_ITEMS_PK
Execution Plan Operations
FILTER
Read a set of row data and discard some rows based on various criteria. To
determine the criteria, operations from a second input may need to be
performed. Input: rows to be examined and, sometimes, an additional
subordinate operation that must be performed for each row from the first input
in order to evaluate criteria. Output: the rows from the first input that met the
criteria.
VIEW
Build a physical representation of a database view or subset of a database
view. Input: set of row data. Output: set of row data that implements the view
or subset of the view.
Notes on Execution Plan Operations
 The optimizer rewrites subqueries as joins and merges them into the main
query whenever possible.
 If a subquery is completely independent of the main query and cannot be
merged into the main query, the optimizer may treat the subquery as a
separate statement and leave it out of the execution plan for the main
query.
 The optimizer expands view definitions and merges them into the main
query wherever possible. A VIEW operation will only appear in an execution
plan when the view definition could not be merged.
Execution Plan Example #5
SELECT /*+ RULE */ a.cust_name, b.contact_name
FROM customers a, contacts@sales.acme.com b
WHERE UPPER(b.contact_name) = UPPER(a.cust_name);
Execution Plan
------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: RULE
1 0 MERGE JOIN
2 1 SORT (JOIN)
3 2 REMOTE* SALES.ACME.COM
4 1 SORT (JOIN)
5 4 TABLE ACCESS (FULL) OF 'CUSTOMERS'
Use EXPLAIN PLAN and TKPROF To Tune Your Applications viii
3 SERIAL_FROM_REMOTE SELECT "CONTACT_NAME"
FROM "CONTACTS" "B”
Execution Plan Operations
REMOTE
Submit a SQL statement to a remote database via Oracle Net. Input: typically
no subordinate operations. Output: the results of the query from the remote
database. Note that the database link used to access the remote database and
the actual SQL submitted to the remote database will be accessible from the
execution plan.
SORT JOIN
Same as the SORT GROUP BY operation, except that the input is sorted by the
join column or columns in preparation for a join using the merge join algorithm.
MERGE JOIN
Perform a join between two sets of row data using the merge join algorithm.
Inputs: two separate sets of row data. Output: the results of the join.
Oracle reads rows from both inputs in an alternating fashion and merges
together matching rows in order to generate output. The two inputs are
assumed to be sorted on the join column or columns.
Summary of Operations
We have not covered all of the execution plan operations, but we have covered
some of the most common ones:
- TABLE ACCESS FULL
- TABLE ACCESS BY INDEX ROWID
- INDEX UNIQUE SCAN
- INDEX RANGE SCAN
- NESTED LOOPS
- NESTED LOOPS OUTER
- HASH JOIN
- MERGE JOIN
- FILTER
- VIEW
- REMOTE
- SORT ORDER BY
- SORT GROUP BY
- SORT JOIN
Elements of a TKPROF Report
 Report heading
– TKPROF version, date run, sort option, trace file
 One entry for each distinct SQL statement in trace file
– Listing of SQL statement
– OCI call statistics: count of parse, execute, and fetch calls, rows
processed, and time and I/O used
– Parse information: parsing user, recursive depth, library cache
misses, and optimizer mode
– Row source operation listing
– Execution plan listing (optional)
– Wait event listing (optional)
Use EXPLAIN PLAN and TKPROF To Tune Your Applications ix
 Report Summary
– OCI call statistics totals
– Counts of how many statements were found in the trace file, how
many were distinct, and how many were explained in the report.
Sample TKPROF Report Heading
TKPROF: Release 8.1.6.1.0 - Production on Wed Aug 9 19:06:36 2000
(c) Copyright 1999 Oracle Corporation. All rights reserved.
Trace file: example.trc
Sort options: default
************************************************************************
count = number of times OCI procedure was executed
cpu = cpu time in seconds executing
elapsed = elapsed time in seconds executing
disk = number of physical reads of buffers from disk
query = number of buffers gotten for consistent read
current = number of buffers gotten in current mode (usually for update)
rows = number of rows processed by the fetch or execute call
Sample OCI Call Statistics
SELECT table_name
FROM user_tables
ORDER BY table_name
call count cpu elapsed disk query current rows
------- ------ -------- ---------- --------- --------- --------- ---------
Parse 1 0.01 0.02 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 14 0.59 0.99 0 33633 0 194
------- ------ -------- ---------- --------- --------- --------- ---------
total 16 0.60 1.01 0 33633 0 194
Misses in library cache during parse: 1
Optimizer goal: CHOOSE
Parsing user id: RSCHRAG [recursive depth: 0]
What the TKPROF Sample on the Previous Slide Tells Us
 The application called on Oracle to parse this statement once while SQL
trace was enabled.
 The parse took 0.01 CPU seconds, 0.02 elapsed seconds.
 No disk I/Os or buffer gets took place during the parse, suggesting that no
misses in the dictionary cache.
 Oracle was called on to execute this statement once.
 The execution took under 0.01 CPU seconds.
 No disk I/Os or buffer gets took place during the execution. (Queries often
defer the work to the fetch phase.)
 Oracle was called on 14 times to perform a fetch, and a total of 194 rows
were returned.
 Fetching took 0.59 CPU seconds, 0.99 elapsed seconds.
Use EXPLAIN PLAN and TKPROF To Tune Your Applications x
 Fetching required 33,633 buffer gets in consistent mode, but no physical
reads were required.
 The statement was not in the library cache (shared pool) when the parse
call came in.
 The cost-based optimizer and a goal of “choose” were used to parse the
statement.
 The RSCHRAG user was connected to the database when the parse
occurred.
 This statement was executed directly by the application; it was not invoked
recursively by the SYS user or a database trigger.
Sample Row Source Operation Listing
Rows Row Source Operation
------- ---------------------------------------------------
194 SORT ORDER BY
194 NESTED LOOPS
195 NESTED LOOPS OUTER
195 NESTED LOOPS OUTER
195 NESTED LOOPS
11146 TABLE ACCESS BY INDEX ROWID OBJ$
11146 INDEX RANGE SCAN (object id 34)
11339 TABLE ACCESS CLUSTER TAB$
12665 INDEX UNIQUE SCAN (object id 3)
33 INDEX UNIQUE SCAN (object id 33)
193 TABLE ACCESS CLUSTER SEG$
387 INDEX UNIQUE SCAN (object id 9)
194 TABLE ACCESS CLUSTER TS$
388 INDEX UNIQUE SCAN (object id 7)
Sample Execution Plan Listing
Rows Execution Plan
------- ---------------------------------------------------
0 SELECT STATEMENT GOAL: CHOOSE
194 SORT (ORDER BY)
194 NESTED LOOPS
195 NESTED LOOPS (OUTER)
195 NESTED LOOPS (OUTER)
195 NESTED LOOPS
11146 TABLE ACCESS (BY INDEX ROWID) OF 'OBJ$'
11146 INDEX (RANGE SCAN) OF 'I_OBJ2' (UNIQUE)
11339 TABLE ACCESS (CLUSTER) OF 'TAB$'
12665 INDEX (UNIQUE SCAN) OF 'I_OBJ#' (NON-UNIQUE)
33 INDEX (UNIQUE SCAN) OF 'I_OBJ1' (UNIQUE)
193 TABLE ACCESS (CLUSTER) OF 'SEG$'
387 INDEX (UNIQUE SCAN) OF 'I_FILE#_BLOCK#' (NON-UNIQUE)
194 TABLE ACCESS (CLUSTER) OF 'TS$'
388 INDEX (UNIQUE SCAN) OF 'I_TS#' (NON-UNIQUE)
Notes About TKPROF Execution Plan Listings
 Execution plans are only included in TKPROF reports if the explain=
parameter is specified when TKPROF is invoked
 TKPROF will create and drop its own plan table if one does not already exist
Use EXPLAIN PLAN and TKPROF To Tune Your Applications xi
 The row counts on each step are actuals—not estimates. This can be very
helpful when troubleshooting queries that perform poorly.
 When TKPROF runs the EXPLAIN PLAN statement for a query, a different
execution plan could be returned than was actually used in the traced
session.
TKPROF Reports: More Than Just Execution Plans
 Listing of SQL statements and library cache miss information helps you
determine if applications are using Oracle’s shared SQL facility effectively.
 Parse, execute, and fetch call counts help you determine if applications are
using Oracle APIs effectively.
 CPU and I/O statistics help you zero in on resource-intensive SQL
statements.
 Row counts on individual steps of the execution plans help you rework
inefficient execution plans.
Wrapping Up
 Use EXPLAIN PLAN, queries against v$sql_plan, the autotrace facility in
SQL*Plus, or GUI tools to view execution plans.
 Use TKPROF to format SQL trace files for human readability.
 Execution plans and TKPROF reports give the DBA and application
developer a wealth of information that can be used to make applications
efficient and perform well.
 The catch: you need to know how to interpret execution plans and TKPROF
reports in order to get any benefit from them. You also ought to know when
to use EXPLAIN PLAN versus when to query v$sql_plan.
Resources. Reference:www.dbspecialists.com/presentations


Regards,
Sagar Maram