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Oracle:Commit Enhancements in Oracle 10g Database Release 2

2011-09-23 15:21 661 查看

Commit Enhancements in Oracle 10g Database Release 2

In Oracle 10g Release 2 the 
COMMIT
command has been enhanced with the 
WRITE
clause to give a degree of control over the way redo information is written to the redo logs during the commit operation. This can improve performance, but it should only be used for processes that meet the following criteria:

They result in large numbers of transactions that require redo log writes.

Data loss can be tolerated in the event of an instance crash during the process.

Waiting for redo log writes is a significant part of the waits associated with the process.

The available options for the 
COMMIT
command and the 
WRITE
clause are displayed below.


COMMIT;
COMMIT WRITE WAIT;
COMMIT WRITE NOWAIT;
COMMIT WRITE BATCH;
COMMIT WRITE IMMEDIATE;


The meanings of the 
WRITE
clause values are listed below.

IMMEDIATE
- The commit "prods" the LGWR process by sending a message, so that the redo is written imemdiately to the redo logs.

BATCH
- The writes to the redo logs are buffered.

WAIT
- The commit command is synchronous. It doesn't return until the relevant redo information is written to the online redo log.

NOWAIT
- The commit command is asynchronous. It can return before the relevant redo information is written to the online redo log.

The action associated with the regular 
COMMIT
command is defined by the 
COMMIT_WRITE
parameter, which accepts a comma-separated list of values.


COMMIT_WRITE = '{IMMEDIATE | BATCH},{WAIT |NOWAIT}'


The 
COMMIT_WRITE
parameter can be specified at instance or session level using the 
ALTER SYSTEM
and 
ALTER SESSION
commands respectively.


ALTER [SYSTEM | SESSION] SET COMMIT_WRITE='WAIT';
ALTER [SYSTEM | SESSION] SET COMMIT_WRITE='NOWAIT';
ALTER [SYSTEM | SESSION] SET COMMIT_WRITE='IMMEDIATE';
ALTER [SYSTEM | SESSION] SET COMMIT_WRITE='BATCH';
ALTER [SYSTEM | SESSION] SET COMMIT_WRITE='BATCH,WAIT';
ALTER [SYSTEM | SESSION] SET COMMIT_WRITE='BATCH,NOWAIT';
ALTER [SYSTEM | SESSION] SET COMMIT_WRITE='IMMEDIATE,WAIT';
ALTER [SYSTEM | SESSION] SET COMMIT_WRITE='IMMEDIATE,NOWAIT';


The default actions for the 
COMMIT_WRITE
parameter and 
WRITE
clause are the same, although at the time of writing the 
COMMIT_WRITE
documentation incorrectly says they are not, so refer to the COMMIT documentations, which says:

"If you specify neither 
WAIT
nor 
NOWAIT
, then 
WAIT
is the default. If you specify neither 
IMMEDIATE
nor 
BATCH
, then 
IMMEDIATE
is the default."

The following code examples show the enhanced commit processing in action. First we define a table for the code to populate.


CREATE TABLE commit_test (
id           NUMBER(10),
description  VARCHAR2(50),
CONSTRAINT commit_test_pk PRIMARY KEY (id)
);


Next we see the variations of the 
WRITE
clause in action. The code truncates the table and measures the time taken to populate it with a commit for each insert. This process is repeated for each variant of the 
WRITE
clause. All the times are measured in hundredths of a second.


SET SERVEROUTPUT ON
DECLARE
PROCEDURE do_loop (p_type  IN  VARCHAR2) AS
l_start  NUMBER;
l_loops  NUMBER := 1000;
BEGIN
EXECUTE IMMEDIATE 'TRUNCATE TABLE commit_test';

l_start := DBMS_UTILITY.get_time;
FOR i IN 1 .. l_loops LOOP
INSERT INTO commit_test (id, description)
VALUES (i, 'Description for ' || i);

CASE p_type
WHEN 'WAIT' THEN COMMIT WRITE WAIT;
WHEN 'NOWAIT' THEN COMMIT WRITE NOWAIT;
WHEN 'BATCH' THEN COMMIT WRITE BATCH;
WHEN 'IMMEDIATE' THEN COMMIT WRITE IMMEDIATE;
END CASE;
END LOOP;
DBMS_OUTPUT.put_line(RPAD('COMMIT WRITE ' || p_type, 30) || ': ' || (DBMS_UTILITY.get_time - l_start));
END;
BEGIN
do_loop('WAIT');
do_loop('NOWAIT');
do_loop('BATCH');
do_loop('IMMEDIATE');
END;
/
COMMIT WRITE WAIT             : 129
COMMIT WRITE NOWAIT           : 86
COMMIT WRITE BATCH            : 128
COMMIT WRITE IMMEDIATE        : 128

PL/SQL procedure successfully completed.

SQL>


Next we see the variations of the 
COMMIT_WRITE
parameter in action. This example follows the format of the previous example, but the 
COMMIT_WRITE
parameter is altered for each run and a standard commit is issued.


SET SERVEROUTPUT ON
DECLARE
PROCEDURE do_loop (p_type  IN  VARCHAR2) AS
l_start  NUMBER;
l_loops  NUMBER := 1000;
BEGIN
EXECUTE IMMEDIATE 'ALTER SESSION SET COMMIT_WRITE=''' || p_type || '''';
EXECUTE IMMEDIATE 'TRUNCATE TABLE commit_test';

l_start := DBMS_UTILITY.get_time;
FOR i IN 1 .. l_loops LOOP
INSERT INTO commit_test (id, description)
VALUES (i, 'Description for ' || i);
COMMIT;
END LOOP;
DBMS_OUTPUT.put_line(RPAD('COMMIT_WRITE=' || p_type, 30) || ': ' || (DBMS_UTILITY.get_time - l_start));
END;
BEGIN
do_loop('WAIT');
do_loop('NOWAIT');
do_loop('BATCH');
do_loop('IMMEDIATE');
do_loop('BATCH,WAIT');
do_loop('BATCH,NOWAIT');
do_loop('IMMEDIATE,WAIT');
do_loop('IMMEDIATE,NOWAIT');
END;
/
COMMIT_WRITE=WAIT             : 141
COMMIT_WRITE=NOWAIT           : 90
COMMIT_WRITE=BATCH            : 78
COMMIT_WRITE=IMMEDIATE        : 94
COMMIT_WRITE=BATCH,WAIT       : 139
COMMIT_WRITE=BATCH,NOWAIT     : 78
COMMIT_WRITE=IMMEDIATE,WAIT   : 133
COMMIT_WRITE=IMMEDIATE,NOWAIT : 87

PL/SQL procedure successfully completed.

SQL>

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