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.
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.
Please let me know if I missed a memo.
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] 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
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.
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.
Regards
Jonathan Lewis
On Sun, 29 Nov 2020 at 19:55, Lok P <loknath.73@xxxxxxxxx> 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?