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Don Burleson Blog

Difference between chained rows and migrated rows

Oracle Database Tips by Donald BurlesonApril 31, 2015

Question: What is the difference between chained rows and migrated rows?

Answer: Originally in Oracle7 there was no distinction, and the terms chained rows and migrated rows were used interchangeably. However, some later nit-pickers say that there is a subtle distinction:

Chained rows - a chained rows is a LOB row (usually a BLOB, CLOB, RAW or LONG RAW) where the row length is large than the data block size. Many shops will create a 32k blocksize to store large columns without row chaining.
Migrated rows - Migrated rows occur when an UPDATE DML causes the rows to expand onto another data block. This can be avoided by setting PCTFREE to a large enough value to accommodate row expansion, and existing migrated rows can be fixed by reorganizing the tables with the dbms_redefinition utility.

Also, note that chained rows and migrated rows only degrade performance for single block fetches, and they will not impact the performance of full table scan operations.

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