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  Oracle Database Tips by Donald Burleson

Monitoring Tables for Chained Rows  

Row chaining occurs as data is added to an existing record. When there is insufficient room for the addition, the row is chained to another block and added there. Block chaining occurs when a row is too long to fit into a single block (such as with long raw or LOB columns or when a row has more than 255 columns). If chaining is occurring regularly, it can lead to significant performance degradation. This degradation is caused by the requirement to read multiple blocks to retrieve a single record. An example of a script to monitor a single table for chained rows is shown in Source 10.10. Au: Confirm Source number here: the next number in sequence is 10, not 11, but based on subsection below, it's difficult to determine if you intended calling this source out of sequence. Fixed Note that this script is limited, in that the table must have a primary or unique key defined in order for it to work. With a companion script, all tables in an application can be checked with this script. As an alternative, you can analyze the tables of concern and use the table statistics report, as described in the following subsection.

The ANALYZE Command  

The ANALYZE command can also be used to generate chained row information into the DBA_TABLES view. Actual chained-row rowids can be listed in a separate table if desired. The general format of this command follows:

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