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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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