8 Tuning SQL*Plus
This chapter provides information about how to tune SQL*Plus for better performance. It discusses the following topics:
For information about tuning Oracle Database, see the Oracle Database Performance Tuning Guide.
8.1 About Tracing Statements
You can automatically get a report on the execution path used by the SQL optimizer and the statement execution statistics. The report is generated after successful SQL DML (that is, SELECT, DELETE, UPDATE and INSERT) statements. It is useful for monitoring and tuning the performance of these statements.
SQL*Plus report output may differ for DML if dynamic sampling is in effect.
8.1.1 Controlling the Autotrace Report
You can control the report by setting the AUTOTRACE system variable.
| Autotrace Setting | Result |
|---|---|
|
SET AUTOTRACE OFF |
No AUTOTRACE report is generated. This is the default. |
|
SET AUTOTRACE ON EXPLAIN |
The AUTOTRACE report shows only the optimizer execution path. |
|
SET AUTOTRACE ON STATISTICS |
The AUTOTRACE report shows only the SQL statement execution statistics. |
|
SET AUTOTRACE ON |
The AUTOTRACE report includes both the optimizer execution path and the SQL statement execution statistics. |
|
SET AUTOTRACE TRACEONLY |
Like SET AUTOTRACE ON, but suppresses the printing of the user's query output, if any. If STATISTICS is enabled, query data is still fetched, but not printed. |
To use this feature, you must create a PLAN_TABLE table in your schema and then have the PLUSTRACE role granted to you. DBA privileges are required to grant the PLUSTRACE role. For information on how to grant a role and how to create the PLAN_TABLE table, see the Oracle Database SQL Language Reference.
For more information about the roles and the PLAN_TABLE, see the Oracle Database SQL Language Reference and the AUTOTRACE variable of the SET command.
Note:
SQL*Plus AUTOTRACE does not support switching containers with the ALTER SESSION SET CONTAINER option. Statistical data gathered in this case may be inconsistent.
Table created.Role dropped.create role plustrace;
Role created.grant plustrace to dba with admin option;
Grant succeeded.Grant succeeded.Example 8-1 Creating a PLAN_TABLE
Run the following commands from your SQL*Plus session to create the PLAN_TABLE in the HR schema:
CONNECT HR @$ORACLE_HOME/rdbms/admin/utlxplan.sql
Example 8-2 Creating the PLUSTRACE Role
Run the following commands from your SQL*Plus session to create the PLUSTRACE role and grant it to the DBA:
CONNECT / AS SYSDBA @$ORACLE_HOME/sqlplus/admin/plustrce.sql drop role plustrace;
Example 8-3 Granting the PLUSTRACE Role
Run the following commands from your SQL*Plus session to grant the PLUSTRACE role to the HR user:
CONNECT / AS SYSDBA GRANT PLUSTRACE TO HR;
8.1.3 Statistics
The statistics are recorded by the server when your statement executes and indicate the system resources required to execute your statement. The results include the following statistics.
| Database Statistic Name | Description |
|---|---|
|
recursive calls |
Number of recursive calls generated at both the user and system level. Oracle Database maintains tables used for internal processing. When Oracle Database needs to make a change to these tables, it internally generates an internal SQL statement, which in turn generates a recursive call. |
|
db block gets |
Number of times a CURRENT block was requested. |
|
consistent gets |
Number of times a consistent read was requested for a block |
|
physical reads |
Total number of data blocks read from disk. This number equals the value of "physical reads direct" plus all reads into buffer cache. |
|
redo size |
Total amount of redo generated in bytes |
|
bytes sent through Oracle Net Services to client |
Total number of bytes sent to the client from the foreground processes. |
|
bytes received through Oracle Net Services from client |
Total number of bytes received from the client over Oracle Net. |
|
Oracle Net Services round-trips to/from client |
Total number of Oracle Net messages sent to and received from the client |
|
sorts (memory) |
Number of sort operations that were performed completely in memory and did not require any disk writes |
|
sorts (disk) |
Number of sort operations that required at least one disk write |
|
rows processed |
Number of rows processed during the operation |
The client referred to in the statistics is SQL*Plus. Oracle Net refers to the generic process communication between SQL*Plus and the server, regardless of whether Oracle Net is installed. You cannot change the default format of the statistics report.
For a more complete list of database statistics, see Statistics Descriptions. For more information about the statistics and how to interpret them, see Gathering Database Statistics.
LAST_NAME SALARY JOB_TITLE
------------------------- ---------- -----------------------------------
King 24000 President
De Haan 17000 Administration Vice President
Kochhar 17000 Administration Vice President
Partners 13500 Sales Manager
Russell 14000 Sales Manager
Hartstein 13000 Marketing Manager
6 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2988506077
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 6 | 360 | 6 (17)| 00:00:01 |
|* 1 | HASH JOIN | | 6 | 360 | 6 (17)| 00:00:01 |
|* 2 | TABLE ACCESS FULL| EMPLOYEES| 6 | 204 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| JOBS | 19 | 494 | 2 (0)| 00:00:01 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("E"."JOB_ID"="J"."JOB_ID")
2 - filter("E"."SALARY">12000)
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
10 consistent gets
0 physical reads
0 redo size
706 bytes sent via Oracle Net Services to client
496 bytes received via Oracle Net Services from client
2 Oracle Net Services roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
6 rows processed6 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2988506077
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 6 | 360 | 6 (17)| 00:00:01 |
|* 1 | HASH JOIN | | 6 | 360 | 6 (17)| 00:00:01 |
|* 2 | TABLE ACCESS FULL| EMPLOYEES| 6 | 204 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| JOBS | 19 | 494 | 2 (0)| 00:00:01 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("E"."JOB_ID"="J"."JOB_ID")
2 - filter("E"."SALARY">12000)
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
10 consistent gets
0 physical reads
0 redo size
706 bytes sent via Oracle Net Services to client
496 bytes received via Oracle Net Services from client
2 Oracle Net Services roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
6 rows processedThis option is useful when you are tuning a large query, but do not want to see the query report.
Note:
Your output may vary depending on the server version and configuration.
Example 8-4 Tracing Statements for Performance Statistics and Query Execution Path
If the SQL buffer contains the following statement:
SELECT E.LAST_NAME, E.SALARY, J.JOB_TITLE FROM EMPLOYEES E, JOBS J WHERE E.JOB_ID=J.JOB_ID AND E.SALARY>12000;
The statement can be automatically traced when it is run:
SET AUTOTRACE ON /
Example 8-5 Tracing Statements Without Displaying Query Data
To trace the same statement without displaying the query data, enter:
SET AUTOTRACE TRACEONLY /
8.2 About Collecting Timing Statistics
Use the SQL*Plus TIMING command to collect and display data on the amount of computer resources used to run one or more commands or blocks. TIMING collects data for an elapsed period of time, saving the data on commands run during the period in a timer.
See the TIMING command, and About Tracing Statements for information about using AUTOTRACE to collect statistics.
To delete all timers, enter CLEAR TIMING.
8.3 Tracing Parallel and Distributed Queries
When you trace a statement in a parallel or distributed query, the Execution Plan output depends on the statement you use.
Table created.create table D2_t2 (unique1 number) parallel - (degree 6);
Table created.create unique index d2_i_unique1 on d2_t1(unique1);
Index created.set long 500 longchunksize 500 SET AUTOTRACE ON EXPLAIN SELECT /*+ INDEX(B,D2_I_UNIQUE1) USE_NL(B) ORDERED - */ COUNT (A.UNIQUE1) FROM D2_T2 A, D2_T1 B WHERE A.UNIQUE1 = B.UNIQUE1;
Execution Plan
----------------------------------------------------------
Plan hash value: 107954098
--------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows |Bytes| Cost(%CPU)| Time | TQ |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 26 | 1 (0)| 00:00:01 | | | |
| 1 | SORT AGGREGATE | | 1 | 26 | | | | | |
| 2 | PX COORDINATOR | | | | | | | | |
| 3 | PX SEND QC (RANDOM) | :TQ10001| 1 | 26 | | | Q1,01 | P->S | QC (RAND) |
| 4 | SORT AGGREGATE | | 1 | 26 | | | Q1,01 | PCWP | |
| 5 | NESTED LOOPS | | 1 | 26 | 1 (0)| 00:00:01 | Q1,01 | PCWP | |
| 6 | PX RECEIVE | | | | | | Q1,01 | PCWP | |
| 7 | PX SEND BROADCAST | :TQ10000| | | | | Q1,00 | P->P | BROADCAST |
| 8 | PX BLOCK ITERATOR | | 1 | 13 | 0 (0)| 00:00:01 | Q1,00 | PCWC | |
| 9 | TABLE ACCESS FULL| D2_T2 | 1 | 13 | 0 (0)| 00:00:01 | Q1,00 | PCWP | |
| 10 | PX BLOCK ITERATOR | | 1 | 13 | 2 (0)| 00:00:01 | Q1,01 | PCWC | |
|* 11 | TABLE ACCESS FULL | D2_T1 | 1 | 13 | 2 (0)| 00:00:01 | Q1,01 | PCWP | |
--------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
11 - filter("A"."UNIQUE1"="B"."UNIQUE1")
Note
-----
- dynamic sampling used for this statementStatistics
----------------------------------------------------------
467 recursive calls
27 db block gets
147 consistent gets
20 physical reads
4548 redo size
502 bytes sent via Oracle Net Services to client
496 bytes received via Oracle Net Services from client
2 Oracle Net Services roundtrips to/from client
14 sorts (memory)
0 sorts (disk)
1 rows processedIf consistent gets or physical reads are high relative to the amount of data returned, it indicates that the query is expensive and needs to be reviewed for optimization. For example, if you are expecting less than 1,000 rows back and consistent gets is 1,000,000 and physical reads is 10,000, further optimization is needed.
Note:
You can also monitor disk reads and buffer gets using V$SQL or TKPROF.
Example 8-6 Tracing Statements With Parallel Query Option
To trace a parallel query running the parallel query option:
create table D2_t1 (unique1 number) parallel - (degree 6);
Example 8-7 To monitor disk reads and buffer gets.
SET AUTOTRACE TRACEONLY STATISTICS
The following shows typical results:
8.4 Execution Plan Output in Earlier Databases
Execution Plan output from Oracle Database 9i Release 2 (9.2) or earlier is different.
Each line of the Execution Plan has a sequential line number. SQL*Plus also displays the line number of the parent operation.
The Execution Plan consists of four columns displayed in the following order:
| Column Name | Description |
|---|---|
|
|
Shows the line number of each execution step. |
|
|
Shows the relationship between each step and its parent. This column is useful for large reports. |
|
|
Shows each step of the report. |
|
|
Shows database links or parallel query servers used. |
The format of the columns may be altered with the COLUMN command. For example, to stop the PARENT_ID_PLUS_EXP column being displayed, enter
COLUMN PARENT_ID_PLUS_EXP NOPRINT
The Execution Plan output is generated using the EXPLAIN PLAN command.
When you trace a statement in a parallel or distributed query, the Execution Plan shows the cost based optimizer estimates of the number of rows (the cardinality). In general, the cost, cardinality and bytes at each node represent cumulative results. For example, the cost of a join node accounts for not only the cost of completing the join operations, but also the entire costs of accessing the relations in that join.
Lines marked with an asterisk (*) denote a parallel or remote operation. Each operation is explained in the second part of the report. See Distributed Transactions Concepts for more information on parallel and distributed operations.
The second section of this report consists of three columns displayed in the following order
| Column Name | Description |
|---|---|
|
|
Shows the line number of each execution step. |
|
|
Describes the function of the SQL statement in the OTHER_PLUS_EXP column. |
|
|
Shows the text of the query for the parallel server or remote database. |
The format of the columns may be altered with the COLUMN command.
8.5 About SQL*Plus Script Tuning
Most performance benefit comes from tuning SQL queries executed in a script. This is done with tools like SQL*Plus's AUTOTRACE command. It involves restructuring queries to make best use of the Oracle Database SQL optimizer. For information about Tuning SQL statements, see the Oracle Database Performance Tuning Guide.
The performance gains made by tuning SQL*Plus-specific commands are smaller, but could be important for some applications. The following system variables and commands can influence SQL*Plus performance.
8.5.1 COLUMN NOPRINT
COLUMN NOPRINT turns off screen output and printing of the column heading and all values selected for the column.
It is better to remove an unneeded column from a SELECT then it is to use COLUMN NOPRINT to stop it displaying. Removing the column from the query means the SQL engine does not need to process it, or need to transfer the column data back to SQL*Plus.
8.5.2 SET APPINFO OFF
Sets automatic registering of scripts through the DBMS_APPLICATION_INFO package. Setting APPINFO OFF prevents administrators monitoring the performance and resource usage of scripts.
If many SQL scripts are being called, then turning APPINFO OFF stops internal SQL*Plus calls to the database DBMS_APPLICATION_INFO package.
8.5.3 SET ARRAYSIZE
Sets the number of rows that SQL*Plus will fetch from the database at one time. Valid values are 1 to 5000.
The effectiveness of setting ARRAYSIZE depends on how well Oracle Database fills network packets and your network latency and throughput. In recent versions of SQL*Plus and Oracle Database, ARRAYSIZE may have little effect. Overlarge sizes can easily take more SQL*Plus memory which may decrease overall performance.
8.5.5 SET FLUSH OFF
SET FLUSH OFF enables the operating system to buffer output. ON disables buffering and flushes output to the screen. Any benefit from setting FLUSH either ON or OFF depends on your operating system and data. The gain may be marginal.
Use OFF only when you run a script that does not require user interaction and whose output you do not need to see until the script finishes running.
8.5.6 SET LINESIZE
SET LINESIZE sets the total number of characters that SQL*Plus displays on one line before beginning a new line.
Keep LINESIZE as small as possible to avoid extra memory allocations and memory copying.
However, if LINESIZE is too small, columns that cannot fit next to each other are put on separate lines. This may reduce performance significantly.
8.5.13 SET TRIMOUT ON SET TRIMSPOOL ON
SET TRIMOUT ON or SET TRIMSPOOL ON removes trailing blanks at the end of each displayed or spooled line.
Setting these variables ON can reduce the amount of data written. However, if LINESIZE is optimal, it may be faster to set the variables OFF. The SQL*Plus output line is blank filled throughout the query processing routines so removing the spaces could take extra effort.
8.5.14 UNDEFINE
Deletes substitution variables that you defined either explicitly (with the DEFINE command) or implicitly (with an argument to the START command or COLUMN NEW_VAL|OLD_VAL).
Use the UNDEFINE command to remove unnecessary substitution variables. This can reduce the time taken for any operation that uses '&', new_value or old_value variables.