关于收缩表和表空间的相关概念（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:
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Compaction of data leads to better cache utilization, which in turn leads to better online transaction processing (OLTP) performance.
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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.
段收缩是一个线上的本地化的操作。dml的操作和查询操作在段收缩的数据逐步移动的过程中不收影响。并发的DML操作会在收缩操作的最后，即空间重新分配的时候，被短暂的阻塞，索引在收缩的过程中会被维护并且在操作完成之后仍然是可用的。段收缩不需要额外的磁盘空间。
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.
段收缩能够回收掉高水位线以上和之下不用的空间，与此相反，空间的回收只能回收掉高水位线上不使用的空间。在收缩操作中，默认的数据库会压缩段空间，以适应高水位线并且释放掉回收的空间。
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.
段收缩需要行被重新定位，因此在进行段收缩前必须首先开启行迁移并且使所有基于这些对象rowid的触发器失效。
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:
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IOT mapping tables
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Tables with rowid based materialized views
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Tables with function-based indexes
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SECUREFILE LOBs
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Compressed tables
索引组织映射表
基于rowid的物化视图表
基于函数索引的表
安全文件的LOB
压缩表
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.
你可以收缩表的空间，索引组织表的空间，索引，分区，子分区，物化视图或者物化视图的日志。你可以做这些通过
alter table
alter index
alter materialized view
alter materialized view log ... +shrink space.
Two optional clauses let you control how the shrink operation proceeds:
两个选项可以让你控制如何进行shrink操作
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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.
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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 theCOMPACT clause.
和其他DDL操作一样，段收缩会引起之后的SQL的重新解析，因为游标的失效，除非指定COMPACT选项。
Examples
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;
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