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RE: Which method is more efficient

From: DENNIS WILLIAMS <DWILLIAMS_at_LIFETOUCH.COM>
Date: Thu, 29 May 2003 06:35:30 -0800
Message-ID: <F001.005A57D4.20030529063530@fatcity.com>


Jared - Thanks. Wow, I learn a new trick each day on this list!

Dennis Williams
DBA, 80%OCP, 100% DBA
Lifetouch, Inc.
dwilliams_at_lifetouch.com

-----Original Message-----
Sent: Wednesday, May 28, 2003 6:15 PM
To: Multiple recipients of list ORACLE-L

There are easier ways to test redo generation than mucking about with logminer.

Update only the column that changes and check redo generation:

15:06:09 rsysdevdb.radisys.com - jkstill_at_dv01 SQL> @t1

USERNAME    SID NAME                                                 VALUE
---------- ---- ---------------------------------------- -----------------
JKSTILL      10 redo size                                                0

1 row selected.

461 rows updated.

USERNAME    SID NAME                                                 VALUE
---------- ---- ---------------------------------------- -----------------
JKSTILL      10 redo size                                          117,128

1 row selected.

Update all columns, only a single column has actually changed:

USERNAME    SID NAME                                                 VALUE
---------- ---- ---------------------------------------- -----------------
JKSTILL      10 redo size                                                0


1 row selected.

461 rows updated.

USERNAME    SID NAME                                                 VALUE
---------- ---- ---------------------------------------- -----------------
JKSTILL      10 redo size                                          226,908

1 row selected.

Updating just the changed field is clearly more efficient. I didn't test a comparison
between multiple columns, updating 1 at a time versus all at once, change only
columns with changed data.

Doing so would require all redo and rollback overhead N number of times rather
than just once, N being the number of changed columns.

Below are the scripts used for testing.

Jared




col sid format 999 head 'SID'
col name format a40
col value format 9999,999,999,999 head 'VALUE' col username format a10 head 'USERNAME'

break on username skip 1 on sid skip 1

select

   sess.username,
   stat.sid,
   name.name name,
   stat.value

from v$sesstat stat, v$statname name, v$session sess where

   stat.sid = sess.sid
   and stat.sid = (

      select s.sid
      from v$session s, v$process p
      where p.addr = s.paddr
         and userenv('SESSIONID') = s.audsid
   )
   and stat.statistic# = name.statistic#    and name.name like 'redo size'
order by name

@redo

update redo_test
set tablespace_name = reverse(tablespace_name) /

@redo

rollback;



@redo

update redo_test
set

   owner = owner
, tablespace_name = reverse(tablespace_name)
, table_name = table_name
, pct_used = pct_used
, pct_free = pct_free
, ini_trans = ini_trans
, max_trans = max_trans
, initial_extent = initial_extent
, next_extent = next_extent
, min_extents = min_extents
, max_extents = max_extents
, pct_increase = pct_increase
, freelists = freelists
, num_rows = num_rows
, blocks = blocks
, empty_blocks = empty_blocks
, avg_space = avg_space
, chain_cnt = chain_cnt
, avg_row_len = avg_row_len

/

@redo

rollback;



DENNIS WILLIAMS <DWILLIAMS_at_LIFETOUCH.COM> Sent by: root_at_fatcity.com
 05/28/2003 09:59 AM
 Please respond to ORACLE-L  

        To:     Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
        cc: 
        Subject:        RE: Which method is more efficient


Bryan - If this is a critical issue, I would try it both ways on a test database and use log miner to examine the amount of redo that is generated.
My recollection is that you will find that the redo record records the before and after data for each field. So just updating all fields may generate significantly more redo. But don't trust my recollection on this issue, test it yourself.

Dennis Williams
DBA, 80%OCP, 100% DBA
Lifetouch, Inc.
dwilliams_at_lifetouch.com

-----Original Message-----
Sent: Wednesday, May 28, 2003 10:50 AM
To: Multiple recipients of list ORACLE-L

Hello everyone,

I have a question for the group of which method is more efficient.

To set the stage my company has a process to load part changes from vendors
into the tables in an 8.1.7.4 Oracle database with archiving on and this database has a standby database at disaster recovery site, so nologging is not an option.

There is a discussion going on as to which method is more effective for updating the information in a table. In looking at effectiveness, I am looking at reducing the amount of redo information produced and having the database do the least amount of work.

  1. Method 1 is to update the information only for the fields that have changed, 1 field at a time.
  2. Method 2 is to update the information for all the fields in the record whether they have changed or not, 1 record at a time.

The size of the record is 1843 bytes and the distribution of field sizes:  2 fields varchar2(240).
 1 field varchar2(150)
15 fields varchar2(50)
1 field varchar2(3)

2 fields varchar2(20)
4 fields varchar2(40)
3 fields varchar2(1)
2 fields varchar2(25)

2 fields number(10,2)
1 field number(13,2)
1 field number(1)
1 field number
1 field varchar2(6)
1 field number (17,2)
1 field varchar2(4)
3 fields that are date.

In the past couple of months the average number of fields changed per record
was 3 to 4 fields per record.

Thanks for your help,

Bryan Rodrigues
Oracle DBA
Elcom, Inc.

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Author: 
  INET: Jared.Still_at_radisys.com

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Author: DENNIS WILLIAMS
  INET: DWILLIAMS_at_LIFETOUCH.COM

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Received on Thu May 29 2003 - 09:35:30 CDT

Original text of this message

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