RE: What will be the rollback plan for a table move + index rebuild operation?

Inline.  

From: Lok P [mailto:loknath.73_at_gmail.com] Sent: Wednesday, June 01, 2022 3:57 PM
To: Mark W. Farnham
Cc: Powell, Mark; Oracle L
Subject: Re: What will be the rollback plan for a table move + index rebuild operation?  

Thank You Mark.

So keeping the importance of performance as more than storage space save in mind, it looks like doing table move+rebuild with a default pctfree of -10 seems a safer bet rather than going with pctfree-0 , mainly for tables which are prone to more updates in its lifecycle. And then set it back to the original/non default pctfree post rebuild.

I checked about the data life cycle of the table which we are considering here. Below are three types mainly

First type)

The transaction table here, is going through Avg ~100million INSERTS/day in batch mode (~100 is the insert batch size) and the inserts happen multithreaded which is why INITRANS is kept as non default- 40. And then a couple of the columns( say C1, C2) get updated for all those rows which were populated/inserted initially as null. And then the data gets moved to a default partition(by updating the partition key itself) from where it gets finally deleted/purged based on that date column which is incremental in nature.

RUN (don’t walk) and find the developer. Scan your existing data for the maximum and average lengths of “C1” and “C2” post update. IF the average is not much different than the average, use the maximum. Otherwise, do statistics on the column length to find a length that is big enough 80% of the time. Explain to the developer that they must plug in an “impossible value” of either that maximum length or the 80% of the time long enough length instead of NULL. This will eliminate a potentially catastrophic amount of row movement it seems likely pctfree 10 is too small for and if maximum is used, perhaps you can go pctfree 0 or close.  

OR, put all the initial rows in a completely different table (perhaps what I call an interim table, that you throw away after every batch is whizzed through). Grab the rows from the interim table doing the update on the fly INTO the real destination.  

Also for the index on column C1 the pctfree kept as - 30 and initrans as - 40. Mostly because it's going through concurrent parallel updates for that column. Maximum during the month end process , it can look back for a past month's worth of data and can Update the column-C1 for specific records.

On a test environment for the same table Avg_row_len is noted as 158 and the size of table is ~1.5TB and holds ~1.3billion rows now. It's a list-range composite partitioned table. After Table moved with PCTFREE-10, its size dropped from ~1.5TB to ~320GB. With pctfree-30 the size becomes ~414 GB. So considering this many DML patterns, I am thinking of just ALTER the table and index to pctfree- 10 and then do the subpartition move+ index rebuild. And finally Alter those back to pctfree-30 i.e. as they were currently.

Second Type)

Here the table is range partitioned and all the historical partitions are just readonly and are already compressed too. This table is never updated unless some ad hoc data fix issue comes up. Data is purged using a drop partition. So in this case we are thinking of rebuilding the historical index partitions with pctfree-0 (which is currently set as pctfree-10 for all).  

Sounds good. When you do those ad hoc data fixes, you probably want to do them into a different table per partition and then use partition exchange. Read some version of Tim Gorman’s “Scaling to Infinity” which is the easiest way to understand the concepts and mechanisms.

Third type)

Here the range partitioned table is mainly loaded using batch insert. No update is happening here. And the data deleted/purged using date criteria which is not really a date column. So here we are thinking of performing table move+ index rebuild with default pctfree-0. And then reset the pctfree back to 10.

I would delete by copying the keepers and doing partition exchange. Unless the deletions are a tiny percentage, this will be faster. (Also explained in Tim Gorman’s paper extremely well.)  

On Wed, Jun 1, 2022 at 6:04 PM Mark W. Farnham <mwf_at_rsiz.com> wrote:

When folks write “OLTP” meaning heavy insert/update/delete, there is a need to examine what sort of information is being considered.  

For the transaction log type information, where the initial row may go through a few changes until it is “complete,” then usually on a rebuild (especially if date or rising number is the first partition), most of the older data can be tightly compressed in a rebuild and any periodic “delete” can likely best be done by removing the oldest partition. 0 percent free is often fine or best for this type of information. Leaving free space in blocks for rows that by application system definition are no longer allowed to change is simply wasted space in tables.  

For inventory balance levels you probably need a little bit of room since numbers can grow in length, and if you do real deletes for discontinued items, that can leave holes that might make a block a candidate free block.  

Those are two examples. For an actual system it is well worth considering the life history length of a row in your big tables that are candidates for rebuild to apply Mr. Powell’s good advice most usefully.  

“Smallest” is often best, but as Mr. Powell wrote, there is dynamic tension between range scan size and insert block splits for indexes, so you have to think about the life cycle of each index to do the optimal thing. The method to employ is to focus your energy on things most likely to have the biggest impact on your goals (which I take it are less space and acceptable performance) and let lukewarm defaults take care of the things that are neither obvious nor likely to change the result most.  

Good luck.      

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Lok P Sent: Tuesday, May 31, 2022 3:33 PM
To: Powell, Mark
Cc: Oracle L
Subject: Re: What will be the rollback plan for a table move + index rebuild operation?    

Thank You so much Mark. I was thinking of simply running commands like 'alter table move subpartition XXXX'; and 'alter index XXXX rebuild subpartition XXXX'; to reclaim the space back. But as you pointed out, I saw table subpartition level the pct_free and ini_trans have non default values set both in table and index level. These tables are OLTP kind of tables which will go through heavy insert/update/deletes. So do you mean to say , in such scenarios we should rebuild the table with say default PCTFREE- 10 and then set the pctfree back to non default(as these are currently set) post rebuild? And I hope other non default params like ini_trans etc. will not play any role in this table move and rebuild operation.  

Below is two sample partition and subpartition table/index storage parameters:-

Table subpartition:-

PCT_FREE PCT_USED INI_TRANS MAX_TRANS INITIAL_EXTENT NEXT_EXTENT MIN_EXTENT MAX_EXTENT

30                     40              255        8388608       1048576           1        2147483645

Local subpartition Index:-

PCT_FREE INI_TRANS MAX_TRANS INITIAL_EXTENT NEXT_EXTENT MIN_EXTENT MAX_EXTENT 30 40 255 65536 1048576 1 2147483645

global index:-

PCT_FREE INI_TRANS MAX_TRANS 10 20 255

Table partition :-

PCT_FREE PCT_USED INI_TRANS MAX_TRANS INITIAL_EXTENT NEXT_EXTENT MIN_EXTENT MAX_EXTENT

10                      20           255           8388608         1048576        1    2147483645

global index:-

PCT_FREE INI_TRANS MAX_TRANS 10 20 255  

On Tue, May 31, 2022 at 11:51 PM Powell, Mark <mark.powell2_at_dxc.com> wrote:

LOK, while it is technically possible that reorganizing a table/index can have an adverse performance impact I would not be that concerned. Just check to see if any of the tables in your plan have non-default percent free reservations. For example, if you have a table with a pctfree of 50 you probably want to lower the pctfree, perform the move, and set it back. Otherwise you would create a lot of basically half full blocks that will never use the free space resulting in the need to read more blocks during a full table scan operation. Likewise, an index range scan on the same values used before the table move could also have to visit more block after the rebuild.  

An index rebuild can adversely impact an insert task if after the index rebuild significantly more block splits are required to support the inserts. This issue tends to fix itself with additional running of the insert task.  

Again, I would not be overly concerned, but you should always check the object space usage and parameter values in use before performing maintenance. This should help you in spotting objects that may need special handling.      

Mark Powell

Database Administration

(313) 592-5148    

From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Pap <oracle.developer35_at_gmail.com> Sent: Tuesday, May 31, 2022 12:19 PM
To: Lok P <loknath.73_at_gmail.com>
Cc: Oracle L <oracle-l_at_freelists.org> Subject: Re: What will be the rollback plan for a table move + index rebuild operation?  

You are rebuilding means just dropping and creating the same table and index without any change in structure and data pattern. So how can it affect the performance negatively?  

On Tue, May 31, 2022 at 7:52 PM Lok P <loknath.73_at_gmail.com> wrote:

As we are trying to move forward with table move+index rebuild operation to get some storage space back on a 12.1 version database. Management asks about the rollback plan just in case we see negative performance reading that table/index or while performing DML on the same table , post the table move+index rebuild operation. We don't have an exact lower environment with similar data volume and that too fragmented one to test application. So trying to understand, is there any way out here to get the exact fragmented table and index back in place as it was before?  

Regards

Lok  

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