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Oracle enable row movement tips

Oracle Database Tips by Donald Burleson

 

The "enable row movement" feature of Oracle is somewhat confusing to beginners and this notes describes some times when "enable row movement" is used. 
 

Oracle has several commands to reclaim unused disk space for objects (tables and indexes).  Using the "alter table xxx shrink space compact" command also has the benefit of making full-table scans run faster, as less block accesses are required.  With standard Oracle tables, you can reclaim space with the "alter table shrink space" command:

SQL> alter table mytable enable row movement;
Table altered

SQL> alter table mytable shrink space;
Table altered

 

 

BEWARE: Enable row movement can cause performance problems

 

Using the enable row movement clause can reduce the clustering_factor for a primary access index, causing excessive I/O.  Oracle introduced the sorted gash cluster as a way to keep an index in-order with the table rows, a technique that greatly reduces I/O for common queries. 

 

Before enabling row movement - Rows clustered with primary index

 


After enabling row movement - Extra I/O for index queries

 

Beware that using "enable row movement" can cause Oracle to move rows to discontinuous data blocks, causing a performance problem.  Remember, the physical placement of rows on data blocks can still make a huge difference in query performance.  For complete details of clustering data to speed-up SQL queries, see my book "Oracle Tuning: The Definitive Reference"


Finding tables and indexes for shrinking

 

The Oracle 10g segment advisor will recommend tables that will benefit from shrinking and indexes that require rebuilding (to reclaim space). 

 

When you add the clause "enable row movement" to a create table statement, you are giving Oracle permission to change the ROWID's.  This allows Oracle to condense table rows and make it easier to reorganize tables.  The enable row movement clause in used within these features:

 

  • Alter table xxx shrink space compact - When using Automatic Segment Storage Management (ASSM, a.k.a. bitmap freelists) you can issue the "alter table xxx shrink" command. to compress the table rows into less data blocks, and Oracle moves down the high water mark to release the space.  This makes full-table scans run faster.
     

  • Flashback table - Using the flashback table features requires "enable row movement".
     

  • Table reorganization - An Oracle10g database can reclaim space within data segments online without affecting the ability of end users to access their data. The only thing that must be ensured before using online segment reorganization capability is that the tablespaces have the Automatic Segment Space Management (ASSM) and row movement features enabled. Oracle10g introduces the ability to reclaim space from a segment by shrinking of the segment. Shrinking a segment will make unused space available to other segments in the tablespace and may improve the performance of queries and DML operations.  

The segment shrink is an online operation where the table being shrunk is open to queries and DML while the segment is being shrunk. Additionally, segment shrink is performed in-place. This is a key advantage over performing Online Table Redefinition for compaction and reclaiming space.

 

Finding opportunities for enable row movement

 

With the introduction of the alter table xxx shrink space compact syntax, the DBA gets a powerful tool for effective and easy database space management. However, the DBA needs to know what data segments experience high space waste in order to reclaim free space to the database and shrink segments. This page shows scripts that can be run to identify opportunities for using the "alter table shrink compact" command and you can use the awr_list_seg_block_space.sql script to report percentages of free space for data segments.

 

Caveat:  Using enable row movement can corrupt any Oracle features that rely on ROWID, such as nested tables, and they should be used with caution.

 
If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


 

 

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