Calculate disk space needed for INDEX

guruparan · Apr 8, 2010 8:21AM

Hi,

How to calculate disk space needed for creating an INDEX? Please point me to resources where I can get detail.

For Example:
I need to create an non UNIQUE INDEX on the following columns, a rough size would do.

<PRE>
EVENT_DATE NOT NULL DATE
APPT_DATE_SLOT VARCHAR2(15)
EVENT_STATUS VARCHAR2(4)
</PRE>

The table has some 7,300,000 rows. Let me know how to calculate too.

All helps are greatly appreciated.

Mark D Powell · Apr 8, 2010 9:28AM

A rough estimate of the space the index will need can be made by

adding the expected actual length of each column plus 6 for the rowid plus 2 for the header X number of table rows that will have an entry so

Date are 7 bytes internal + 15 + 4 + 2 + 6 = 34 X 7.3M = 248200000 X overhead for block header, initrans, etc...

we will use 20% for block overhead or 1.2 X 248200000 = 297840000 which is about 285M

The actual allocation will vary depending on your tablespace extent allocation method.

HTH -- Mark D Powell --

448389 · Apr 8, 2010 9:01AM

Hi

dbms_space.create_index_cost can help you

guruparan · Apr 8, 2010 9:14AM

I haven't created yet! But I can still run this proc? BTW, I am in Oracle 9.2

guruparan · Apr 8, 2010 9:18AM

Looks like Oracle 9.2 doesn't have it,

<pre>
SQL> desc dbms_space.create_index_cost
ERROR:
ORA-04043: object dbms_space.create_index_cost does not exist
</pre>

Mark D Powell · Apr 8, 2010 9:28AM

A rough estimate of the space the index will need can be made by

adding the expected actual length of each column plus 6 for the rowid plus 2 for the header X number of table rows that will have an entry so

Date are 7 bytes internal + 15 + 4 + 2 + 6 = 34 X 7.3M = 248200000 X overhead for block header, initrans, etc...

we will use 20% for block overhead or 1.2 X 248200000 = 297840000 which is about 285M

The actual allocation will vary depending on your tablespace extent allocation method.

HTH -- Mark D Powell --

guruparan · Apr 8, 2010 9:30AM

Thanks Mark. That's about it.

Mark D Powell · Apr 8, 2010 10:41AM

You are welcome. Thank you for the points. If you get to build the index soon and can remember post back with the actual results.

-- mark --

448389 · Apr 8, 2010 11:01AM

Yes is it to known the future size BEFORE create index by using statistics

look the Ask Tom http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:1833377500346797242

for psoug this package exist since 7.3.4. I use it on 10.2.0[2-4] and 11g ({ORACLE_HOME}/rdbms/admin/dbmsspu.sql)
http://psoug.org/reference/dbms_space.html

my test also :
SQL*Plus: Release 11.1.0.6.0
[email protected]>create table tsygale as select level num from dual connect by level<=100000;

Table created.
[email protected]>@seg_tab
Enter value for segment_name: TSYGALE
old 3: where segment_name in ('&segment_name')
new 3: where segment_name in ('TSYGALE')

TABLESPACE_NAME SEGMENT_NAME MO
-------------------- ----------------------------------- ----------
SYSTEM TSYGALE 2
[email protected]>exec dbms_stats.gather_table_stats(user,'TSYGALE');

PL/SQL procedure successfully completed.

Elapsed: 00:00:02.95

[email protected]>variable used number
[email protected]>variable alloc number
exec dbms_space.create_index_cost( 'create index isygale on tsygale(object_name,object_type,owner)', :used, :alloc );

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.25
16:56:38 [email protected]>print used

USED
----------
2761720

16:56:49 [email protected]>print alloc

ALLOC
----------
4194304
[email protected]>create index isygale on tsygale(object_name,object_type,owner);

Index created.

Elapsed: 00:00:00.50

[email protected]>@seg_tabind
Segment name ?TSYGALE
old 3: where (segment_name in ('&&segment_name')
new 3: where (segment_name in ('TSYGALE')
old 5: segment_name in (select index_name from dba_indexes where table_name in ('&&segment_name'))
new 5: segment_name in (select index_name from dba_indexes where table_name in ('TSYGALE'))

TABLESPACE_NAME SEGMENT_TYPE SEGMENT_NAME MO
-------------------- ------------------ ----------------------------------- ----------
SYSTEM TABLE TSYGALE 8
SYSTEM INDEX ISYGALE 4

Elapsed: 00:01:22.31

So package estimate to 4 194 304 Octets and after the index created the size is to 4MO, so ....

Edited by: Sygale on Apr 8, 2010 5:02 PM

Jonathan Lewis · Apr 8, 2010 11:10AM

Sygale wrote:

for psoug this package exist since 7.3.4. I use it on 10.2.0[2-4] and 11g ({ORACLE_HOME}/rdbms/admin/dbmsspu.sql)
http://psoug.org/reference/dbms_space.html

Unfortunately the OP says he is using 9.2, and although the package has been around since 7.3 that particular procedure was not introduced until 10g - and it has a couple of flaws built in, see: http://jonathanlewis.wordpress.com/2009/05/22/index-size/

Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk

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448389 · Apr 8, 2010 11:38AM

Jonathan Lewis wrote:
Unfortunately the OP says he is using 9.2, and although the package has been around since 7.3 that particular procedure was not introduced until 10g - and it has a couple of flaws built in, see: http://jonathanlewis.wordpress.com/2009/05/22/index-size/

Sorry for this mistake, have you a method to define on which version a functionality is delivered ?

I read your article, I see some errors in this package,
but it can help us to define if we have enough space on disk to create index or table in Clone environment. Before create it on production

A real test is always better than an estimate test

I recommend to add 10% margin from development to production (the production database is always run

)

Thanks

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