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ibm db2 db 中 reorg 命令的用途是什么？Reorg 在内部做什么？如果在表上创建了新索引，是否需要运行 reorg？

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The REORG operation performs the following functions:

  • Checks referential integrity, if applicable for the target table, and either deletes rows that violate it or invalidates any affected indexes. (Referential integrity is the relational property that each foreign-key value in a table exists as a primary-key value in the referenced table.

  • Performs an internal reorganization of one or more of the indexes for the table (all types) to improve the internal storage of this information and thereby the performance when the index is used to access data. It can rebuild all indexes, selectively rebuild one or more named indexes, or selectively rebuild one or more segments of one or more named indexes.

  • Populates a DEFERRED index that is created with a CREATE INDEX statement. A DEFERRED index is an empty index structure that can be populated at a later date.

  • Rebuilds precomputed views. For example, if precomputed view maintenance is set to OFF in your application, you can use the REORG command to rebuild precomputed views only, without touching the indexes on the target table. Alternatively, you can use the REORG command to rebuild both indexes and views.

In addition to rebuilding aggregate table data, the REORG command rebuilds indexes on aggregate tables.

A REORG operation is necessary in the following cases:

  • To rebuild the affected indexes if you use a database restore operation to restore individual segments of a table or index.

  • Whenever modifications to a database affect more than about 30 percent of the data, run the TMU with a REORG statement for any tables directly modified. Periodically rebuilding such tables and indexes with a REORG statement ensures referential integrity and optimal performance.

  • To reorganize invalid STAR indexes. Certain operations can invalidate STAR indexes. For example, increasing the MAXROWS PER SEGMENT or the MAXSEGMENTS parameter on a table, or using an ALTER statement to expand a segment, can invalidate STAR indexes on tables that reference the altered table. These operations always generate a warning message that says STAR indexes based on the altered table might be invalid, in which case the affected STAR indexes need to be reorganized. You can either reorganize affected indexes when the message is issued or schedule the REORG operation for a more convenient time. However, any non-query (INSERT, UPDATE, or DELETE) operation against a table that has an invalid index results in an error message that says the index must be reorganized. You must perform a REORG operation before the table can be accessed for an INSERT, UPDATE, DELETE, or LOAD operation.

REORG is unnecessary in the following cases:

  • If no changes are made to the database except by complete loads of data.

  • If the table and indexes are segmented alike and new index data is loaded into new index segments corresponding to new table segments.

于 2014-01-10T10:03:18.377 回答
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当您在（至少对于 iSeries）DB2 中删除一条记录（或行）时，它会将记录标记为要删除，但不会实际删除该记录。运行 reorg 时，它会获取那些标记为删除的记录，然后将它们物理删除。我相信它还会移动所有数据记录，以便在此过程中获得最佳性能。在 iSeries 上，可以通过告诉文件（或表）重用删除记录来消除这种需求。

正如我所暗示的，我知道 DB2 for iSeries（或 IBM i）就是这种情况。我只能假设这个过程在 DB2 上是类似的。

于 2010-08-05T18:34:11.667 回答
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它实际上所做的是将物理记录放回主键顺序，并在正确的位置使用正确数量的可用空间（由 PCTFREE 指定）。

显然，这也需要重建索引，从而产生平衡良好的 btree。

于 2010-08-18T01:04:19.903 回答