RE: Re: Column Length modification

sayan wrote:  

NVL(colname,old_col)  

which is brilliant, especially if you have time based partitioning and eventual “unhook partition” purging at some age (like the common 7 years or 25 years for EPA projects). At some point then, no values would exist in old_col and you could drop the wrapper, without ever copying squat, since all your new values go into the “new” colname without changing a line of code.  

mwf  

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Sayan Malakshinov Sent: Monday, March 22, 2021 7:44 AM
To: Lok P
Cc: ahmed.fikri_at_t-online.de; oracle list Subject: Re: Re: Column Length modification  

Hi Lok,  

I would prefer Jonathan's approach or renaming old column and creating new one with the same name, but with new precision and wrapper view with NVL(colname,old_col).

But if you really want to change it, I would change your approach: instead of p.1 "create backup table using CTAS", I would create partitioned table backup_table and use "alter table backup_table switch partition with old_table".

It will make an original table empty, so you don't need p.2.  

On Mon, Mar 22, 2021 at 2:25 PM Lok P <loknath.73_at_gmail.com> wrote:

Thank You So much. So just to avoid missing any grants synonyms and related consequences like invalidation of package/procedure etc. Is it good to just intact the main table but move data in and out , something as below..

1)Create a backup table as "tab1_backup" from the main table TAB1 using CTAS approach using parallelism without creating indexes constraints etc on the backup table.

2)Truncate the main table TAB1.

3)make the indexes UNUSABLE in the main table TAB1.

4)Alter the column in the main table TAB1 to modify the existing column from number(15,5) to number(22,0).

5)Insert data into the main table(tab1) from the backup table(tab1_backup) using direct path load + parallel threads. And in case of unusable indexes this step should be pretty fast.

6)Rebuild the indexes in the main table.  

Regards

Lok      

On Mon, Mar 22, 2021 at 4:16 PM ahmed.fikri_at_t-online.de <ahmed.fikri_at_t-online.de> wrote:

oracle does the same when you add the new colum, update it and remove the old one (so doing this, the table is recreated three times).

just use direct load + parallel + constraints novalidate. This work perfectly even for huge tables

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• Original-Nachricht --- Von: Lok P Betreff: Re: Column Length modification Datum: 22. März 2021, 11:38 An: Jonathan Lewis Cc: Oracle L

Just that, i think the option of creating the new object fully with the data and rename afterwards by dropping the original object may not be a good option if the object which we are trying to alter is in TB's and partitioned, thinking if any other possible way to achieve the same?  

On Sun, Mar 21, 2021 at 8:09 AM Lok P <loknath.73_at_gmail.com> wrote:

Thank you Jonathan.  

Yes it's exactly the same error ORA-01440: which we are encountering. And the column is not having any data with non zero precision. But as we are standardizing the data elements across our applications, we want to now not allow any junks in future and thus trying to fix the precision thing as part of this length modification. This will make things consistent across all our applications and easy for understanding.  

Now if my understanding is correct, the way you are suggesting i..e altering column length as (22,7) (which will not make any ORA-01440 error happen )+ having the check constraint added to the table will technically help us achieving the same thing (without any additional performance overhead) as simply altering the length to number(22,0). But is it something that will create confusion and thus we should keep it clean i.e. column length (22,0) only without any additional constraint? And to achieve that , is the best approach is the one suggested by Ahmed i.e. create the object fully with the new structure(i.e. with number(22,0)) and then drop the old one and rename the new one?  

Regards

Lok  

On Sun, Mar 21, 2021 at 3:30 AM Jonathan Lewis <jlewisoracle_at_gmail.com> wrote:  

If you're see an error then show us exactly what it is.

I assume it's

ORA-01440: column to be modified must be empty to decrease precision or scale  

You're trying to change a column from (15,5) to (22,0) which means you're going to lose 5 decimal places - do any of the rows have data that isn't purely integer, if not are you happy for the values to change as you go from 5d.p. to integer?  

If you need 22 digits precision, and no decimal places you could modify your column to (27,5) and then add a check constraint that says (check colX = trunc(colX)) as a way of ensuring that you don't have any non-integer values. (You could update the table,set colX to trunc(colX) where colX != trunc(colX)before adding the constraint, or you could add the constraint enabled but not validated, then do the update then set the constraint validated. ** or ceiling() or round(), depending on what you think best suits your requirements  

Regards

Jonathan Lewis         

On Sat, 20 Mar 2021 at 19:10, Lok P <loknath.73_at_gmail.com> wrote:

We are using version 11.2.0.4 of Oracle exadata. Our requirement is to modify column length of a table from Number(15,5) to Number(22) and we are seeing errors and its saying to make the column empty before making this modification. So to achieve this we are thinking of doing this in multiple steps like

1. Add new column(COL_new) with number(22,0) to the same table

2)Then update the new column with all the values of original column(say COL1)

3)Then drop the original column(COL1) which is having length number(15,5) 4)Then rename the new column(COL_NEW) to original i.e. COL1.

 We are in the process of doing multiple such modifications to some big partition and non partitioned table. And in this process the Update seems to be a tedious one as it will scan the full table and may lead to row chaining and also drop the existing column and renaming new columns will need the application to stop pointing to this object or else they may fail. Also stats seems to be gathered fully again on the table after this along with if any index pointing to these columns needs to be recreated. So multiple issues highlighted with this process by the team. Want to understand from experts if there exists any better way of achieving this with minimal interruption and in quick time?  

Thanks

Lok  

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Best regards,
Sayan Malakshinov

Oracle performance tuning engineer

Oracle ACE Associate
http://orasql.org

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http://www.freelists.org/webpage/oracle-l Received on Mon Mar 22 2021 - 18:32:41 CET