You use online segment shrink to reclaim fragmented free space below the high water mark in an Oracle Database segment. The benefits of segment shrink are these:
1.Compaction of data leads to better cache utilization, which in turn leads to better online transaction processing (OLTP) performance.
2.The compacted data requires fewer blocks to be scanned in full table scans, which in turns leads to better decision support system (DSS) performance.
Segment shrink is an online, in-place operation. DML operations and queries can be issued during the data movement phase of segment shrink. Concurrent DML operations are blocked for a short time at the end of the shrink operation, when the space is deallocated. Indexes are maintained during the shrink operation and remain usable after the operation is complete. Segment shrink does not require extra disk space to be allocated.
Segment shrink reclaims unused space both above and below the high water mark. In contrast, space deallocation reclaims unused space only above the high water mark. In shrink operations, by default, the database compacts the segment, adjusts the high water mark, and releases the reclaimed space.
Segment shrink requires that rows be moved to new locations. Therefore, you must first enable row movement in the object you want to shrink and disable any rowid-based triggers defined on the object. You enable row movement in a table with the ALTER TABLE ... ENABLE ROW MOVEMENT command.
Shrink operations can be performed only on segments in locally managed tablespaces with automatic segment space management (ASSM). Within an ASSM tablespace, all segment types are eligible for online segment shrink except these:
1.IOT mapping tables
2.Tables with rowid based materialized views
3.Tables with function-based indexes
Before invoking online segment shrink, view the findings and recommendations of the Segment Advisor. For more information, see "Using the Segment Advisor".
You invoke online segment shrink with Oracle Enterprise Manager or with SQL commands in SQL*Plus. The remainder of this section discusses the command line method.
Note:You can invoke segment shrink directly from the Recommendation Details page in Enterprise Manager. (See Figure 19-4.) Or, to invoke segment shrink for an individual table in Enterprise Manager, display the table on the Tables page, select the table, and then click Shrink Segment in the Actions list. (See Figure 19-1.) Perform a similar operation in Enterprise Manager to shrink indexes, materialized views, and so on.
You can shrink space in a table, index-organized table, index, partition, subpartition, materialized view, or materialized view log. You do this using ALTER TABLE, ALTER INDEX, ALTER MATERIALIZED VIEW, or ALTER MATERIALIZED VIEW LOG statement with the SHRINK SPACE clause.
Two optional clauses let you control how the shrink operation proceeds:
1.The COMPACT clause lets you divide the shrink segment operation into two phases. When you specify COMPACT, Oracle Database defragments the segment space and compacts the table rows but postpones the resetting of the high water mark and the deallocation of the space until a future time. This option is useful if you have long-running queries that might span the operation and attempt to read from blocks that have been reclaimed. The defragmentation and compaction results are saved to disk, so the data movement does not have to be redone during the second phase. You can reissue the SHRINK SPACE clause without the COMPACT clause during off-peak hours to complete the second phase.
2.The CASCADE clause extends the segment shrink operation to all dependent segments of the object. For example, if you specify CASCADE when shrinking a table segment, all indexes of the table will also be shrunk. (You need not specify CASCADE to shrink the partitions of a partitioned table.) To see a list of dependent segments of a given object, you can run the OBJECT_DEPENDENT_SEGMENTS procedure of the DBMS_SPACE package.
As with other DDL operations, segment shrink causes subsequent SQL statements to be reparsed because of invalidation of cursors unless you specify the COMPACT clause.
Shrink a table and all of its dependent segments (including BASICFILE LOB segments):
ALTER TABLE employees SHRINK SPACE CASCADE;
Shrink a BASICFILE LOB segment only:
ALTER TABLE employees MODIFY LOB (perf_review) (SHRINK SPACE);
Shrink a single partition of a partitioned table:
ALTER TABLE customers MODIFY PARTITION cust_P1 SHRINK SPACE;
Shrink an IOT index segment and the overflow segment:
ALTER TABLE cities SHRINK SPACE CASCADE;
Shrink an IOT overflow segment only:
ALTER TABLE cities OVERFLOW SHRINK SPACE;
Oracle Database SQL Language Reference for the syntax and restrictions of the ALTER TABLE, ALTER INDEX, ALTER MATERIALIZED VIEW, and ALTER MATERIALIZED VIEW LOG statements with the SHRINK SPACE clause
Oracle Database SecureFiles and Large Objects Developer's Guide for more information about LOB segments
When you deallocate unused space, the database frees the unused space at the unused (high water mark) end of the database segment and makes the space available for other segments in the tablespace.
Prior to deallocation, you can run the UNUSED_SPACE procedure of the DBMS_SPACE package, which returns information about the position of the high water mark and the amount of unused space in a segment. For segments in locally managed tablespaces with automatic segment space management, use the SPACE_USAGE procedure for more accurate information on unused space.
See Also:Oracle Database PL/SQL Packages and Types Reference contains the description of the DBMS_SPACE package
ALTER TABLE table DEALLOCATE UNUSED KEEP integer; ALTER INDEX index DEALLOCATE UNUSED KEEP integer; ALTER CLUSTER cluster DEALLOCATE UNUSED KEEP integer;
The KEEP clause is optional and lets you specify the amount of space retained in the segment. You can verify that the deallocated space is freed by examining the DBA_FREE_SPACE view.
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