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Row Chaining & Row Migration

Posted by FatDBA on January 5, 2013

Row Chaining:
Occurs when a row is too large to fit into an EMPTY data block.In this Oracle stores the DATA for the row in a CHAIN of one or more Data BLOCKS. Chainining occurs when row is Inserted or Updated and happens with rows that contains big data types i.e. LOB etc.
Example: suppose if you have DB_BLOCK_SIZE of 8k (standard) and you want to insert a row of more than 8K in to a Block, Oracle then uses other DB block to fit the remaining portion of the row to it and this is known as Row Chaining and always happens when a row is being INSERTED.

ora_row_chained_1

Row Migration:
Migrated row on the other hand is a row which has been updated larger than it initially was – and if as a result it doesn’t fit into its original block, the row itself is moved to a new block, but the header (kind of a stub pointer) of the row remains in original location. This is needed so that any indexes on the table would still be able to find that row using original ROWIDs stored in them).
Migration always happens in case of UPDATE.

ora_row_migration_1

To Identify Row Chaing and Row Migration:
1. USER_TABLES
2. Using view V$SYSSTAT
3. Using Analyze

How to avoid and eliminate Chained/Migrated rows ?
====================================================
1-ALTER TABLE … MOVE command.
You can MOVE the table by using ALTER TABLE … MOVE statement that enables you to relocate data of a non partitioned table or of a partition of a partitioned table into a new segment, and optionally into a different tablespace for which you have quota. This statement also lets you modify any of the storage attributes of the table or partition, including those which cannot be modified using ALTER TABLE. You can also use the ALTER TABLE … MOVE statement with the COMPRESS keyword to store the new segment using table compression.

Note : Moving a table changes the row ids of the rows in the table. This causes indexes on the table to be marked UNUSABLE,and DML accessing the table using these indexes will receive an ORA-01502 error. The indexes on the table must be dropped or rebuilt. Likewise, any statistics for the table become invalid and new statistics should be collected after moving the table.

2-Increase PCTFREE.
The PCTFREE parameter sets the minimum percentage of a data block to be reserved as free space for possible updates to rows that already exist in that block. For example, assume that you specify the following parameter within a CREATE TABLE statement. If the PCTFREE has been set to a low value, that means there is not enough room in the block for updates. To avoid migration,all tables that are updated should have their PCTFREE set so that there is enough space within the block for updates.You need to increase PCTFREE to avoid migrated rows. If you leave more free space available in the block for updates, then the row will have more room to grow.

3- Import/Export can be taken as an approach to eliminate the migrated rows.

4- Avoid to create tables with more than 255 columns.

 

——- T.E.S.T ——–

SQL> @$ORACLE_HOME/RDBMS\ADMIN\utlchain.sql
Table Created

SQL> Desc CHAINED_ROWS;

Name                                                     Null?                         Type
—————————————– ——– —————————-
OWNER_NAME                                                      VARCHAR2(30)
TABLE_NAME                                                         VARCHAR2(30)
CLUSTER_NAME                                                   VARCHAR2(30)
PARTITION_NAME                                               VARCHAR2(30)
SUBPARTITION_NAME                                      VARCHAR2(30)
HEAD_ROWID                                                       ROWID
ANALYZE_TIMESTAMP                                      DATE

SQL> SELECT name, value FROM v$sysstat WHERE name = ‘table fetch continued row’;

no rows selected

SQL> create table chain_row_test (a varchar(4000),b varchar(4000), c varchar(4000));
Table created.

SQL> insert into chain_row_test (a,b,c) values ( 1, rpad(‘*’,40000000,’*’), rpad(‘*’,2300000,’*’) )
1 row created.

SQL> commit ;
Commit complete.

SQL> SELECT count(*) FROM chained_rows;

COUNT(*)
———-
0

SQL> analyze table chain_row_test list chained rows into chained_rows ;
Table analyzed.

SQL> SELECT count(*) FROM chained_rows;

COUNT(*)
———-
1

SQL> select dbms_rowid.rowid_block_number(rowid) from chain_row_test;

DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID)
————————————
508711

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