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RE: sequential read on full-table scan?

From: Christian Antognini <Christian.Antognini_at_trivadis.com>
Date: Mon, 23 May 2005 12:02:33 +0200
Message-ID: <7F0C000A3ABA6241A10C9ABF37EEB46D0A4A78@MSXVS01.trivadis.com>

>>Chained or migrated rows It is a problem if you see many db file
>>sequential read waits against a table when the execution plan of the
>>SQL statement calls for a full table scan. This indicates the table
>>has many chained or migrated rows. Oracle goes after each chained or
>>migrated row with the single-block I/O call.
>
>IMO this is not true.

A clarification... In case of migrated rows the additional I/O isn't = performed because the row header is skipped. The addition I/O, i.e. the = WAIT event, is present in the tracefile if a row is chained and 1) it is = not chained in the block itself (more than 255 columns) 2) the chained = block has already been read and it's still in the buffer cache. If a row = is migrated and chained at the same time, the "rules" for chained rows = applies.

Chris

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Received on Mon May 23 2005 - 06:07:19 CDT

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