Re: Row migrated in new data blocks

<xhoster_at_gmail.com> wrote in message
news:20060818122641.848$wH_at_newsreader.com...
> "Mark D Powell" <Mark.Powell_at_eds.com> wrote:
>>
>> Being that the question was in reference to a full table scan the
>> migrated row header is ignored and the row piece is read when the full
>> scan reads the block that contains the migrated row piece. This block
>> could be read before or after the block that contains the migrated row
>> header, that is, the original block that contained the row before it
>> was migrated.
>
> OK, but now let's say that Oracle, for some reason, needs the rowid of the
> row (say, because "rowid" is in the select list). The rowid of the row
> would be, AFAICT, the rowid of the head-piece, not the rowid of the
> data-piece. Once the FTS finds the data-piece, it obviously knows the
> rowid of the data-piece. But how does it figure out the rowid of the
> corresponding head-piece? Does the data pieces have a reverse pointer,
> going back to the head-piece?
>
> Xho
>
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The migrated row is flagged as 'no-header', and its entry includes a 'head rowid' - i.e. a pointer back to where the row originally came from.

One reason for this is that you may update the row again in a way that makes it small enough to migrate back to the original block.

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Regards

Jonathan Lewis
http://www.oracle.com/technology/community/oracle_ace/ace1.html#lewis

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