Re: Table with ~255+ columns

Mark,

You missed a memo - Oracle can MIGRATE a row back to its original location on an update: https://jonathanlewis.wordpress.com/2014/02/10/row-migration/ The note is a bit old, so I really ought to check it for newer versions, I guess.

Oracle MIGHT also do something sensible with rows that are chained because of column lengths when the number of columns is no more than 255, but anything might happen if there are more than 255 non-null columns in the row.

Regards
Jonathan Lewis

On Mon, 30 Nov 2020 at 20:57, Mark W. Farnham <mwf_at_rsiz.com> wrote:

> I’m not aware of Oracle doing anything like repatriating migrated rows or
> row pieces due to just an update in any version of Oracle.
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> I tend to think that would require the instantiation of a deity to code
> correctly and I can think of dozens of optimizations that would be of
> greater utility to the user base. Moving segments was provided to do that
> functionality in a bulletproof and relatively simple way, so I doubt we
> will ever see it.
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> Please let me know if I missed a memo.
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>
> *From:* oracle-l-bounce_at_freelists.org [mailto:
> oracle-l-bounce_at_freelists.org] *On Behalf Of *Jonathan Lewis
> *Sent:* Monday, November 30, 2020 6:03 AM
> *To:* Lok P
> *Cc:* Oracle L
> *Subject:* Re: Table with ~255+ columns
>
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> The answer to that question may be dependent on the version of Oracle and
> on how the row got to the state it was in before the update to null of the
> 276th column.
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> The first point to consider, though, is that the row HAS to be in at least
> two pieces if *any *column after the 255th is non-null, so updating just
> the 276th isn't likely to have any significant effect. Given that you're
> on 11.2.0.4 I doubt if Oracle will do anything nice all you do is set that
> column to null - in fact it may simply find another way of doing something
> nasty, but you'd have to do some experiments to find out what that could be.
>
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> Regards
>
> Jonathan Lewis
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> On Sun, 29 Nov 2020 at 19:55, Lok P <loknath.73_at_gmail.com> wrote:
>
> One thing i saw, few of the stats gather statements on this table are
> logging heavy statistics "table fetch continued row" on this table, And
> another thing i notice, as per the NULL/NOT NULL data pattern i published
> from one of the sample partition, it shows out of total 277 columns , we
> have 276th position column populated with NOT NULL value for almost all the
> rows, so its confirm that we must be having the rows split into two parts
> as we have 276th column inserted as NOT NULL. And , as we have the row size
> restricted to ~avg ~277bytes, so my thought was , at max it will intra row
> chaining considering we are not updating the columns in this table. But
> then I observed in old partitions that the 276th position column is UPDATED
> to NULL value for all of those rows, so in that case will Oracle try to
> readjust them into a single piece again, or it will be in the same two
> rowpiece only?
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>
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>

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