Understanding SQL Trace, TKPROF and
Execution Plan for beginners
Carlos Sierra

Agenda
• Motivation
• SQL Trace
• TKPROF
• Execution Plan

Motivation
• My report is taking too long…
• One Materialize View is sudently hanging...
• This page is taking long since last upgrade...
• It runs faster on my development instance...
• Everything is slow, then the DBA has to fix it...
• Where to start?

SQL Trace
• Performance information on individual SQL
• CPU and Elapsed Time
• Wait Events
• Execution Plan
• Rows processed
• Call count (parse, execute and fetch)
• Physical and Logical reads

Tracing Process
1. Turn SQL Trace on
2. Execute some SQL or PL/SQL
3. Turn SQL Trace off
4. Find Trace File
5. Aggregate Trace into a Summary report (TKPROF)
6. Review Trace and TKPROF

Turning SQL Trace on and off
• DBMS_SESSION.SESSION_TRACE_ENABLE
– WAITS (default TRUE)
– BINDS (default FALSE)
– PLAN_STAT (default NULL)
• NEVER
• FIRST_EXECUTION (equivalent to NULL)
• ALL_EXECUTIONS
• DBMS_SESSION.SESSION_TRACE_DISABLE

Other ways to turn Trace on and off
• DBMS_SESSION.SET_SQL_TRACE(TRUE|FALSE)
• ALTER SESSION SET SQL_TRACE = TRUE|FALSE
• ALTER SESSION SET EVENT 10046
• DBMS_MONITOR.SESSION_TRACE_ENABLE or DIS
• ALTER SYSTEM SET EVENTS 'sql_trace [sql:&&sql_id]
bind=true, wait=true';
• ORADEBUG
• Oracle Enterprise Manager (OEM)

Alter Session
• ALTER SESSION SET
– SQL_TRACE = [ TRUE | FALSE ]
– EVENTS ‘10046 trace name context forever, level N’
• N = 1: all executions (10g-) or 1st execution (11g+)
• N = 4: Binds
• N = 8: Waits
• N = 12 = 4 + 8
• N = 16: each execution (11g+)
• N = 64: 1st + each where DB time > 1min (11.2.0.2+)

Finding SQL Trace
• Get Trace Filename from V$DIAG_INFO (11g+)
• Get Trace Directory from USER_DUMP_DEST (10g-)
– Use V$DIAG_INFO on 11g and 12c

Sample SQL over Sample HR Schema

GettingSQLTrace

MOS39817.1

TKPROF
• Inputs a SQL Trace
• Outputs an aggregated TKPROF report
• Example:
– tkprof trace_file.trc tkprof.txt
• Sort (optional):
– sort=(exeela,fchela)

GettingaTKPROF

• Rows returned on 1st execution, in average and max
• Consistent reads, physical reads/writes, etc.

• Metrics include aggregates of sub-operations
• Rows processed (returned by operations)

• Wait events (enabled by default on API used)
• Useful to understand gap between CPU and Elapsed

Execution Plan
• Set of steps to execute a SQL statement
– Mostly Access Operations and Joins
• Better understood by correlating Plan to SQL
• Hard to read without predicate information
• Column projection also eases understanding
• Figuring out execution order can be a challenge!

Common Access Operations
• Table access
– Full
– By ROWID
• Index access
– Full
– Range
– Unique

Join Types
• Equijoin
• Outer-join
• Semi-join
• Anti-join
• Cross-join
• Self-join

Name the Subquery
In-line View
Scalar SQ
Non-correlated SQ
Correlated SQ

Join Methods
• Nested Loop
– Small sets
– Index access
• Hash Join
– Medium to large sets
– Equality predicates
• Sort Merge Join

Nested Loop

Hash Join

Sort Merge Join

Getting Predicates and Projection
• Use DBMS_XPLAN
• DISPLAY (from PLAN_TABLE)
– FORMAT: ADVANCED
• DISPLAY_CURSOR (from Memory thus actual plan)
– FORMAT: ADVANCED ALLSTATS LAST
• SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY%());

Two
Tables
Join

Five Tables Join

e2(6) ▻ e1(7) ▻ jh(8) ▻ d(12) ▻
l(14)
1. e2 ▻ e1 : hj(3)
2. ▻ jh : nl(2)
3. ▻ d : nl(1) semi pp
4. ▻ l : nl(11)

SQLDeveloper

TKPROF

Summary
• Trace shows performance information per SQL
• Trace and TKPROF do not require additional license
• Trace includes Execution Plan when Cursor closes
• DBMS_XPLAN provides Predicates and Projection
• Understand first what the SQL does
• Then correlate SQL to Execution Plan
• Focus where the Time is spent!

References
• My Oracle Support (MOS)
– 39817.1 Interpreting Raw SQL_TRACE output
– 376442.1 How to Collect 10046 Trace (SQL_TRACE)
Diagnostics for Performance Issues
• https://oracle-base.com/articles/misc/sql-trace-
10046-trcsess-and-tkprof
• Oracle Documentation
• Google

Questions

The End
• http://carlos-sierra.net/
– Blog and downloads
• carlos.sierra.usa@gmail.com
– Questions and follow-up
• Keep learning!