Re: Big Update/DML

Hi Sanjay,

Depending on your needs, load and other circumstances, I'd consider also one more old approach: rename table, rename and make that column invisible, create view with nvl(stamp_process_calc_invisible, processed_calc_amt) as stamp_process_calc, create instead of trigger, so user could insert/change that invisible column, and started update process in parts. Then you will need another short downtime to drop view and rename table and column back

чт, 27 авг. 2020 г., 3:43 Sanjay Mishra <smishra_97_at_yahoo.com>:

> Sayan
>
> Update statement is
>
> Update snows.stamp_detail set set stamp_process_calc=processed_calc_amt;
>
> Tried to use
> 1. Parallel DML with 100 --> Taking 20+hrs
> 2. CTAS was tried using half a billion as well as 1 billion rows with
> parallel 50, 75,100 - Almost same result
> 3. CTAS with nologging using same step 2 but still not much improvement
>
> We have 5-10 such big table and so running each with this much time-frame
> need high downtime
> Tx
> Sanjay
>
> On Wednesday, August 26, 2020, 08:23:18 PM EDT, Sayan Malakshinov <
> xt.and.r_at_gmail.com> wrote:
>
>
> Hi Sanjay,
>
> It would be better if you provide more details about your update. Exact
> update statement would be helpful. is this column nullable/not null?
>
>
> On Thu, Aug 27, 2020 at 1:12 AM Sanjay Mishra <dmarc-noreply_at_freelists.org>
> wrote:
>
> Andy
>
> Yes look like is an option if we are doing work online and despite take
> more time but need not require downtime. In our case multiple DDL are
> running to existing environment due to Application upgrade and so all work
> has to be done with downtime. So challenge is reduce time of DML operations
> on big tables containing few billions rows.
>
> Tx
> Sanjay
>
> On Wednesday, August 26, 2020, 11:20:55 AM EDT, Andy Sayer <
> andysayer_at_gmail.com> wrote:
>
>
> It does sound like a virtual column could be the ideal solution. But if
> data needs to be physically stored or cannot be calculated
> deterministically at any point in time then Connor has a great demo of
> using dbms_redefinition to create a new table online with a function to map
> the new column. There’s obviously some overhead with context switching but
> it may be far better than some of the obstacles you might be facing at the
> moment:
>
> https://connor-mcdonald.com/2016/11/16/performing-a-large-correlated-update/
> (and you might be able to help it with pragma udf in the right
> circumstances).
>
> Obviously, how helpful this is depends where the work is currently going
> and how online this needs to be.
>
>
> Thanks,
> Andrew
>
> On Wed, 26 Aug 2020 at 16:00, Jonathan Lewis <jlewisoracle_at_gmail.com>
> wrote:
>
>
> Is that 3-4 billion rows each, or total ?
>
> I would be a little suspicious of an update which populates a new column
> with a value derived from existing columns. What options might you have for
> declaring a virtual column instead - which you could index if needed.
>
> Be extremely cautious about calculating space requirements - if you're
> updating every row on old data might you find that you're causing a
> significant fraction of the rows in each block to migrate, and there's a
> peculiarity of bulk row migration that can effectively "waste" 25% of the
> space in every block that becomes the target of a migrated row.
>
> This effects can be MUCH work when the table is compress (even for OLTP)
> since the update has to decompress the row before updating and then only
> "re-compresses" intermittently as the block becomes full. The CPU cost can
> be horrendous and you still have the problem of migration if the addition
> means the original rows can no longer fit in the block.
>
> If it is necessary to add the column you may want to review "alter table
> move online" can do in the latest versions (in case you can make it add the
> column as you move) or review the options for dbms_redefinition - maybe
> running several redefinitions concurrently rather than trying to do any
> parallel update to any single table.
>
> Regards
> Jonathan Lewis
>
>
>
>
>
>
>
>
>
> --
> Best regards,
> Sayan Malakshinov
> Oracle performance tuning engineer
> Oracle ACE Associate
> http://orasql.org
>

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