Shrinking Database Segments Online

Shrinking Database Segments Online


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:


Compaction of data leads to better cache utilization, which in turn leads to better online transaction processing (OLTP) performance.


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 operation 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:


IOT mapping tables


Tables with rowid based materialized views


Tables with function-based indexes


See Also:
Oracle Database SQL Reference for more information on the ALTER TABLE command.
Invoking Online Segment Shrink


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 Enterprise Manager (EM) 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 EM. (See Figure 14-4.) Or, to invoke segment shrink for an individual table in EM, display the table on the Tables page, select the table, and then click Shrink Segment in the Actions list. (See Figure 14-1.) Perform a similar operation in EM 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. Refer to Oracle Database SQL Reference for the syntax and additional information on shrinking a database object, including restrictions.


Two optional clauses let you control how the shrink operation proceeds:


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.


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.


Examples


Shrink a table and all of its dependent segments (including LOB segments):


ALTER TABLE employees SHRINK SPACE CASCADE;


Shrink a 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;