Log space involved in Index Rebuilding

Hello everyone..
I read in one site that, an index rebuild requires space greater than or equal to the table on which it is defined and as a . Is that true?
Already log grows when the index was first created right? Why does does it grows again when the index is rebuilt?
-- Thanks and Regards Srikar Reddy Gondesi, Trainee SQL Server Database Administrator Miracle Software systems, Inc.

Hi,
I will start fro the second point: A Log File is managed using a virtual blocks named VLF (Virtual Log Files). A VLF is a basic unit whci can not get bigger or smaller after it is created. Increasing the log file or decreasing
it, is done by adding or removing VLFs. You can not remove a VLS until all it content has been truncated.
Each action is written in the log file according to the database backup mode (simple/bulk/full) and the action itself as mentioned in post above (check Andrew Bainbridge post above). therefor
if you do an action that need to write the log file it will not use the older place and you need to use new place (place on VLF which is in unused status or build new VLF). You can get more information on managing the Log File in this great presentation:
http://cdn.prod.brentozar.com/wp-content/uploads/2012/09/How-SQL-Server-Stuff-Works-Log-Files.pdf
or an Hebrw short blog: http://ariely.info/Blog/tabid/83/EntryId/130/Transaction-Log-File-Structure.aspx
About the first point, i am not sure i understood your meaning, but if you asking if index get more place on disk then original table then the answer is basically no (but an index add to the original disk place and might use the same
size or so). We can decide what information (column) is kept in the index Principle
To clarify: If the question is related just to "what/when is written in the log file" then the answer of Andrew covers it. If the question "What/when causes
the file to grow" then you need to read more on the VLF as mentioned here (written to log file do not necessarily grow the file size, in a well manage system theoretically the file should never grow/decrease).
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Similar Messages

Index rebuild

Hi Team,
Please tell me any difference b/w alter index <index> rebuild or alter index <index> rebuild move.
Is there any difference that more space will be released if I move index to diffrenet tablespace and move move back to orginal tablespace.
I know that if we use move option we can move index to different tablespace,
Regards,
jit

Even if you would do lots of dml's, that's not certainly means that you need the rebuilding of the index. If your assertion is based on this that there would be some "holes" within the leaf blocks which wont be reused and would require a rebuild, it's wrong then since Oracle would take care of these things on its own. So again, what criteria, other than an OLTP db you used to conclude that you need rebuilding of indexes?
But I just wanted to know If I go for alter index rebuild or alter index rebuild move and move back the indexes, which option will be best for me mean by which option I can reclaim more space.Well, I am sorry to say but I think you really don't know very well both the commands. The commands don't differ in the space reclaim given by them but in the way they do the rebuild. Please read the documentation to know what both commands do and how? BTW, from where you picked the command alter index rebuild move since I am not sure that such command exists?
Aman....

Amount of temporary tablespace size used for index rebuild

Hi All,
I want to know approximate amount of temporary tablespace size used for index rebuild. I need this information to avoid the insufficient temporary tablespace error during the huge index rebuild.
Is there a query or procedure to find it out.
Thank you.

Hi,
While creating the index, the temporary segment is created in the permanent tablespace. So permanent tablespace must have sufficient space.
http://www.oracle-base.com/articles/10g/SpaceObjectTransactionManagement10g.php
http://aprakash.wordpress.com/2010/01/05/numeric-segment-name/
Anand

Index Rebuild in Oracle 7

Hi DBA's,
DB Details: ORACLE 7 , Machine : P-III : VMWARE..
I am getting an error in prod like follows,
24-may-2011 04:24:33 am: start rebuilding the index on the table ...
alter index
ERROR at line 1:
ORA-01652: unable to extend temp segment by 168168 in tablespace SFX_INDEX
doubt:
why does the error say "extend temp segment" for the permanent index tablespace.
We have 3000MB Free space in Same tablespace & we have 2000MB free space in Temp also.But Still it is Giving error in "ORACLE 7 "
Any Idea...
Help me soon .

user11919409 wrote:
DB Details: ORACLE 7 , Machine : P-III : VMWARE..
I am getting an error in prod like follows,
24-may-2011 04:24:33 am: start rebuilding the index on the table ...
alter index
ERROR at line 1:
ORA-01652: unable to extend temp segment by 168168 in tablespace SFX_INDEX
doubt:
why does the error say "extend temp segment" for the permanent index tablespace. Because when you rebuild an index, the new index structure starts life as a temp segment (and so can be easily dropped in case of failure) and gets renamed and reassigned as the index segment once the index rebuild process is complete.
>
We have 3000MB Free space in Same tablespace & we have 2000MB free space in Temp also.But ; Still it is Giving error in "ORACLE 7 "
It's the free space in the SFX_INDEX tablespace that's the problem, you don't have 168168 free contiguous blocks within the tablespace necessary for the index (the actual amount of space this corresponds to is of course is dependent on your block size).
Cheers
Richard Foote
http://richardfoote.wordpress.com/

Oracle Sys table index rebuild

hi, expert
I am new to this expert Forums
Blow Table Index are rebuild,This table are relation with sys user or not?
after rebuild index, getting error in alert log file
ORA-00600: internal error code, arguments: [psdnop-1], [1410], [], [], [], [], [], [], [], [], [], []
ORA-01410: invalid ROWID
when trying to select * from dba_network_acls getting error massage Rowid invalid
ALTER INDEX CTXSYS.SYS_C003684 REBUILD;
ALTER INDEX CTXSYS.SYS_C003683 REBUILD;
ALTER INDEX CTXSYS.SYS_C003682 REBUILD;
ALTER INDEX CTXSYS.SYS_C003681 REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_54056 REBUILD;
ALTER INDEX CTXSYS.SYS_C003677 REBUILD;
ALTER INDEX CTXSYS.SYS_IL0000054052C00002$$ REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_54044 REBUILD;
ALTER INDEX CTXSYS.SYS_C003675 REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_54038 REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_54031 REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_54028 REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_54018 REBUILD;
ALTER INDEX CTXSYS.DR_STOPLIST_PK REBUILD;
ALTER INDEX CTXSYS.DR_STOPLIST_UNIQ REBUILD;
ALTER INDEX CTXSYS.DR_THS_BT REBUILD;
ALTER INDEX CTXSYS.DR_UNIQ_THS_BT REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_54003 REBUILD;
ALTER INDEX CTXSYS.DR_THS_RINGID REBUILD;
ALTER INDEX CTXSYS.DR_UNIQ_THS_PHRASE REBUILD;
ALTER INDEX CTXSYS.SYS_C003629 REBUILD;
ALTER INDEX CTXSYS.SYS_C003626 REBUILD;
ALTER INDEX CTXSYS.SYS_C003627 REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_53991 REBUILD;
ALTER INDEX CTXSYS.SYS_IOT_TOP_53963 REBUILD;
ALTER INDEX OLAPSYS.SYS_C004604 REBUILD;
ALTER INDEX OLAPSYS.SYS_C004600 REBUILD;
ALTER INDEX XDB.SYS_C003740 REBUILD;
ALTER INDEX XDB.SYS_C003741 REBUILD;
ALTER INDEX XDB.SYS_C003742 REBUILD;
ALTER INDEX MDSYS.UNIQUE_CLASS_CONSTRAINT REBUILD;
ALTER INDEX MDSYS.UNIQUE_USER_CONSTRAINT REBUILD;
ALTER INDEX MDSYS.UNIQUE_LRS_GEOM_LAYER REBUILD;
ALTER INDEX MDSYS.UNIQUE_NETWORK_LAYER REBUILD;
ALTER INDEX MDSYS.UNIQUE_NETWORK_LINK_LAYER REBUILD;
ALTER INDEX MDSYS.UNIQUE_NETWORK_NODE_LAYER REBUILD;
ALTER INDEX MDSYS.UNIQUE_NETWORK_PATH_LAYER REBUILD;
ALTER INDEX MDSYS.UNIQUE_NETWORK_PATH_LINK_LAYER REBUILD;
It is possible to recreate this index again to solve this problem, or any other solution.
Thanks in Advance sir,

Pl do not post duplicate threads - Oracle sys Index rebuild then Getting error Invalid Error Ora-01410

On Index Rebuild in 10g

My partitioned table has 150 Million records with average row length of 444. I need to add 8 million more records from another non partitioned table, which will fit into 2 partitions of earlier table.
I have set database to no force logging, disabled all foreign keys and using APPEND hint to add the rows. I suppose this won't generate redo logs for table.
Now to reduce redo logs due to indexes, I am planning to set them unusable and then skip the unusable indexes (any unique index will be in used state as Oracle needs to enforce integrity). I can do this for partitioned indexes and rebuild required partitions alone.
But I have around 12 global single column(with maxlength of 25 bytes) indexes . so my question is, if I rebuild the index offline (as online rebuild scans the full table- Doc ID: 272762.1), will it scans the full table or the existing index, which is in unusable state?
Or is there any other approach? I suppose I can try using partition exchange option but I still need to deal with those indexes.

Rebuilding unusable Index scans the table rather than the original index. Also we cannot build it online.
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:1669403535933#11145054166877

Redo log space requests VALUE high

SELECT name||' = '||value
FROM v$sysstat
WHERE name = 'redo log space requests';
I am noticing 40+ space requests for some of my Oracle 9.2.0.5 databases.
On another 7.3.4 DB I see this over 140 but this DB shutdown only on weekends so this cumulative value increases I presume.
I have 20MB of 5 groups already. Do I still add another 2 more groups or increase their sizes ?
I did read somewhere that I'd have to increase the log_buffer parameter. So how do we deal with this issue ? Any repercussions if I let this as it is for now ?
The cause of this would be redo logs are not big enough or otherwise ?
Thanks.

user4874781 wrote:
Thanks for your response Charles.
So if I understand this correctly ... Redolog Space requests corresponds to a either an incorrectly sized redo log file / DBWR / CKPT needs to be tuned.
Maybe I was interpreting this the wrong way. (Possibly)
" The statistic 'redo log space requests' reflects the number of times a user process waits for space in the redo log buffer. " If that is the case, if there was longer waits for this event, I was under the assumption that log_buffer needs to be increased.
http://www.idevelopment.info/data/Oracle/DBA_tips/Tuning/TUNING_6.shtml
* Yes, the waits have increased to 70 as of now (since 40 yesterday .. DB was started Saturday night and will run till weekend) Less activity as of now, since the day has just started ; so it would definitely rise by end of the day. I took a look at the above article, and I think I understand why the article is slightly confusing. With due respect to the author, the article was last modified 16-Apr-2001, which I believe is before the Oracle documentation was better clarified regarding these statistics. From:
http://download.oracle.com/docs/cd/B14117_01/server.101/b10755/stats002.htm
"redo log space requests: Number of times the active log file is full and Oracle must wait for disk space to be allocated for the redo log entries. Such space is created by performing a log switch. Log files that are small in relation to the size of the SGA or the commit rate of the work load can cause problems. When the log switch occurs, Oracle must ensure that all committed dirty buffers are written to disk before switching to a new log file. If you have a large SGA full of dirty buffers and small redo log files, a log switch must wait for DBWR to write dirty buffers to disk before continuing."
"redo log space wait time: Total elapsed waiting time for "redo log space requests" in 10s of milliseconds"
It is quite possible that the "redo log space requests" will increase with every redo log file switch, which should not be too much of a concern. You may want to focus a little more on the "redo log space wait time" statistic, which indicates how much wait time was involved waiting. You might also want to focus on the system-wide wait event interface, examining how the accumulated wait time increases from one sampling of each of the statistics to the next.
* I have 1 log switch every 11 minutes ; BTW ; I have 5 log groups of 20 MB each as of now. So I am assuming 40 MB of 4 or 5 log groups should be fine as per your suggestion ?If you have the disk space, considering that it is an ancient AIX box, you may want to set the redo log files to an even larger size, possibly 100MB (or larger). You may then want to force periodic switches of the redo log, for instance once an hour, or once every 30 minutes.
* This is an ancient AIX box with 512 MB Ram. Is the redo log located on a fast device ? I'd have to find that out ( any hints on that ? )
* Decreasing the log_buffer is possible on weekends since I'd have to bounce it for it to take effect.
I will increase the log files accordingly and hopefully the space waits will reduce. Thanks again.Someone else, possibly Sybrand, on the forums might be familiar with AIX and be able to provide you with an answer. If you are seeing system-wide increasing wait time for redo related waits, then that might be an indication that the redo logs are not located on a fast device.
Charles Hooper
IT Manager/Oracle DBA
K&M Machine-Fabricating, Inc.

High redo log space wait time

Hello,
Our DB is having very high redo log space wait time :
redo log space requests 867527
redo log space wait time 67752674
LOG_BUFFER is 14 MB and having 6 redo logs groups and the size of redo log file is 500MB for each log file.
Also, the amount of redo generated per hour :
START_DATE START NUM_LOGS MBYTES DBNAME
2008-07-03 10:00 2 1000 TKL
2008-07-03 11:00 4 2000 TKL
2008-07-03 12:00 3 1500 TKL
Does increasing the size of LOG_BUFFER will help to reduce the redo log space wait ?
Thanks in advance ,
Regards,
Aman

Looking quickly over the AWR report provided the following information could be helpful:
1. You are currently targeting approx. 6GB of memory with this single instance and the report tells that physical memory is 8GB. According to the advisories it looks like you could decrease your memory allocation without tampering your performance.
In particular the large_pool_size setting seems to be quite high although you're using shared servers.
Since you're using 10.2.0.4 it might be worth to think about using the single SGA_TARGET parameter instead of the specifying all the single parameters. This allows Oracle to size the shared pool components within the given target dynamically.
2. You are currently using a couple of underscore parameters. In particular the "_optimizer_max_permutations" parameter is set to 200 which might reduce significantly the number of execution plans permutations Oracle is investigating while optimizing the statement and could lead to suboptimal plans. It could be worth to check why this has been set.
In addition you are using a non-default setting of "_shared_pool_reserved_pct" which might no longer be necessary if you are using the SGA_TARGET parameter as mentioned above.
3. You are using non-default settings for the "optimizer_index_caching" and "optimizer_index_cost_adj" parameters which favor index-access paths / nested loops. Since the "db file sequntial read" is the top wait event it might be worth to check if the database is doing too excessive index access. Also most of the rows have been fetched by rowid (table fetch by rowid) which could also be an indicator for excessive index access/nested loop usage.
4. You database has been working quite a lot during the 30min. snapshot interval: It processed 123.000.000 logical blocks, which means almost 0.5GB per second. Check the top SQLs, there are a few that are responsible for most of the blocks processed. E.g. there is a anonymous PL/SQL block that has been executed almost 17.000 times during the interval representing 75% of the blocks processed. The statements executed as part of these procedures might be worth to check if they could be tuned to require less logical I/Os. This could be related to the non-default optimizer parameters mentioned above.
5. You are still using the compatible = 9.2.0 setting which means this database could still be opened by a 9i instance. If this is no longer required, you might lift this to the default value of 10g. This will also convert the REDO format to 10g I think which could lead to less amount of redo generated. But be aware of the fact that this is a one-way operation, you can only go back to 9i then via a restore once the compatible has been set to 10.x.
6. Your undo retention is set quite high (> 6000 secs), although your longest query in the AWR period was 151 seconds. It might be worth to check if this setting is reasonable, as you might have quite a large undo tablespace at present. Oracle 10g ignores the setting if it isn't able to honor the setting given the current Undo tablespace size.
7. "parallel_max_servers" has been set to 0, so no parallel operations can take place. This might be intentional but it's something to keep in mind.
Regards,
Randolf
Oracle related stuff:
http://oracle-randolf.blogspot.com/
SQLTools++ for Oracle:
http://www.sqltools-plusplus.org:7676/
http://sourceforge.net/projects/sqlt-pp/

Oracle sys Index rebuild then Getting error Invalid Error Ora-01410

Oracle sys Index rebuild,
after Getting error Invalid Error Ora-01410 in alert log file.every Insert ,update and delete.
Oracle Database 11g Enterprise Edition Release 11.1.0.7.0 - 64bit Production
PL/SQL Release 11.1.0.7.0 - Production
Oracle sys Index rebuild list
CTXSYS
EXFSYS
FLOWS_030000
FLOWS_FILES
MDSYS
OLAPSYS
ORDSYS
OWBSYS
SI_INFORMTN_SCHEMA
TSMSYS
WKSYS
MDSYS
After rebuild above index im getting error while inserting,update,delete table rows
Error like 'INVALID ROWID' alert log file
ANY ONE HELP ME TO RESOLVE THE PROBLEM
Edited by: 927230 on May 26, 2012 8:51 PM
Edited by: 927230 on May 26, 2012 8:53 PM

927230 wrote:
Oracle sys Index rebuild after Getting error Invalid Error Ora-01410
Oracle 11.1 version
Oracle sys Index rebuild list
CTXSYS
EXFSYS
FLOWS_030000
FLOWS_FILES
MDSYS
OLAPSYS
ORDSYS
OWBSYS
SI_INFORMTN_SCHEMA
TSMSYS
WKSYS
MDSYS
After rebuild above index im getting error while inserting,update,delete table rows
Error like 'INVALID ROWID'
ANY ONE HELP ME TO RESOLVE THE PROBLEMwhich came first the error or the rebuild?
COPY & PASTE whole session & SQL so we can see exactly what you do & how Oracle responds
post results from following SQL below
SELECT * FROM V$VERSION

Goldengate Extracts reads slow during Table Data Archiving and Index Rebuilding Operations.

We have configured OGG on a near-DR server. The extracts are configured to work in ALO Mode.
During the day, extracts work as expected and are in sync. But during any dialy maintenance task, the extracts starts lagging, and read the same archives very slow.
This usually happens during Table Data Archiving (DELETE from prod tables, INSERT into history tables) and during Index Rebuilding on those tables.
Points to be noted:
1) The Tables on which Archiving is done and whose Indexes are rebuilt are not captured by GoldenGate Extract.
2) The extracts are configured to capture DML opeartions. Only INSERT and UPDATE operations are captured, DELETES are ignored by the extracts. Also DDL extraction is not configured.
3) There is no connection to PROD or DR Database
4) System functions normally all the time, but just during table data archiving and index rebuild it starts lagging.
Q 1. As mentioned above, even though the tables are not a part of capture, the extracts lags ? What are the possible reasons for the lag ?
Q 2. I understand that Index Rebuild is a DDL operation, then too it induces a lag into the system. how ?
Q 3. We have been trying to find a way to overcome the lag, which ideally shouldn't have arised. Is there any extract parameter or some work around for this situation ?

Hi Nick.W,
The amount of redo logs generated is huge. Approximately 200-250 GB in 45-60 minutes.
I agree that the extract has to parse the extra object-id's. During the day, there is a redo switch every 2-3 minutes. The source is a 3-Node RAC. So approximately, 80-90 archives generated in an hour.
The reason to mention this was, that while reading these archives also, the extract would be parsing extra Object ID's, as we are capturing data only for 3 tables. The effect of parsing extract object id's should have been seen during the day also. The reason being archive size is same, amount of data is same, the number of records to be scanned is same.
The extract slows down and read at half the speed. If normally it would take 45-50 secs to read an archive log of normal day functioning, then it would take approx 90-100 secs to read the archives of the mentioned activities.
Regarding the 3rd point,
a. The extract is a classic extract, the archived logs are on local file system. No ASM, NO SAN/NAS.
b. We have added "TRANLOGOPTIONS BUFSIZE" parameter in our extract. We'll update as soon as we see any kind of improvements.

Alter index rebuild question

Hello experts,
I have a simple question regarding rebuilding indexes...
First, I am running 11.2.0.2 on Solaris 10.
My reason for rebuilding is to relocate the indexes to their own tablespace, and while I recognize this is a subjective reason as far as performance is concerned, it makes administration easier.
That being said, my question is in regards to use of "compute statistics" and also "online", and "nologging".
Since I do want to minimize he impact to our users, and this table is one of the most used tables in the database, my syntax was going to be as follows:
alter index <schema>.<index> rebuild online nologging tablespace <ts_idx> compute statistics;
Questions:
1.   does doing the rebuild online preclude queries against the table during the duration of the rebuild?
2.   does use of nologging improve speed of rebuilding
3.   is it advisable to include the "compute statistics". I believe I read on Jonathan Lewis' page that statistics are gathered automatically and to include "compute statistics" was not necessary when rebuilding an index.
And I guess one final question is whether I can include all the options in the same rebuild statement as I have shown above?
I've tested the above in our test database, but it seems to hang when using the online option (with all the other options).
Thanks in advance.

Excellent resource. Thanks for the link. While I've read most of those discussions on either Jonathan's or Richard's web sites already, it is a good presentation and reference.
Unfortunately, it does not answer either of the questions that I asked. The reason I asked those questions is because the answers you gave contradict what i read in Oracle documentation (although it might be version specific).
Also, it does seem to be slightly faster to rebuild using nologging (in my test environment).
I've tested this with multiple indexes and the timing is slightly improved consistently.
{quote}
SQL> ALTER INDEX ARADMIN."T2179_C1_0944_1037" REBUILD LOGGING TABLESPACE REMEDY_IDX;
Index altered.
Elapsed: 00:00:10.68
SQL> ALTER INDEX ARADMIN."T2179_C1_0944_1037" REBUILD nologging TABLESPACE REMEDY_IDX;
Index altered.
Elapsed: 00:00:08.50
{quote}
The use of online, however, seems to "hang" for some reason, even in my test database while no one else is logged into the database except me.
Referencing Oracle's documentation, it says that rebuilding an index online does not allow queries against the base table. This is why I asked this question, but your answer is different than Oracle's. In this case, however, it is for Oracle 10.2, so not sure if this is still true for 11.2.
Ref. Oracle Text SQL Statements and Operators
[ONLINE]
ONLINE enables you to continue to perform updates, inserts, and deletes on a base table; it does not enable you to query the base table.
You cannot use PARALLEL with ONLINE. ONLINE is only supported for CONTEXT indexes.
Now, regarding the use of "compute statistics", I am pretty sure I recall reading either in Jonathan's papers, or in Richard's papers, that this is not necessary and/or is redundant. But then why would Oracle include this as an option. This is counter intuitive, which again is why I'm asking. Do you know if any reference that can clear this up?

Alter index rebuild reverse duration

Hi,
I'm working on an Oracle Database 11g Release 11.1.0.6.0 - 64bit Production With the Real Application Clusters option.
I would need to rebuild reverse 2 indexes, 60gb each.
Is it possibile to forecast somehow the opreation duration?
The alter index rebuild is locking (insert on those tables are locked), doesn't it?
Thanks in advance,
Samuel

You can rebuild it online, but it will demand extra temporary space, and can take more time.
ALTER INDEX INDX REBUILD ONLINE

Estimating archive log space

Hi All,
My Oracle server is a 10gR2
Currently oracle is set in Non Archive mode .
Now i need to put the same in ARCHIVE mode for that i need to estimate unix space required for archive log storage.
Plan is to keep archive for a day and back up at EOD and location wil be cleaned for next day.
Activty planned for a day is to load around 86 GB of data to tables and this will create around 80 GB of index as well .
Can any one help me to estimate Archive log space required for this activity for a day.
Thanks

Hi Satish,
Currently i have 120 Gb archive space .. But when i did a test last time load failed in the middle with following error.
ERROR:
ORA-00257: archiver error. Connect internal only, until freed.
Thats why i want to exactly estimate archive space requirement ..before start loading again .
Now regarding data i load is plain ascii files .
Do we have any queries or a method with which we can estimate?
Thanks

Index rebuild - temporary segment

Hi,
I am getting an error while rebuilding the indexes, like unable to allocate extents in temporary segment. Kindly help me to solve this.
Regards,
Mathew Collins

You should never drop an index with the intention of recreating it without first checking the size of the index against the size of the available temp tablespace sort area.
If you are using an alter index rebuild to reorganize the index then unless you specify a new tablespace in the command the existing tablespace must have enough free space to hold the entire original index and the rebuilt index. An alter index rebuild also uses temporary (sort) tablespace so you still need to ensure that your index has not outgrown your temp tablespace.
HTH -- Mark D Powell --

Different b/w index rebuild and index rebuild online

hi..guys could u plz tel me difference between index rebuild and index rebuild online

There is no difference in both the commands. Both will rebuild the index structure from the scratch.But in the first case with only Rebuild, as long as the index, its temporary segment is not prepared and merged together, index is not available for the other users for use. The Online clause makes the index available for others even while being rebuild.
Rebuilding index online has the same concept of creating them online to some extent,
http://download.oracle.com/docs/cd/B10501_01/server.920/a96521/indexes.htm#3062
HTH
Aman....

Log space involved in Index Rebuilding

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