Automatic Undo Retention (32)

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本文介绍了Oracle Database 10g中如何自动调整回滚段保留时间参数，即undoretention周期。文章详细解释了该周期如何根据undo表空间的类型（自动扩展或固定大小）及使用情况进行调整，以确保最长的保留期同时避免空间不足的情况。

Oracle Database 10g automatically tunes a parameter called the undo retention
period.The undo retention period indicates the amount of time that must pass before old
undo information—that is, undo information for committed transactions—can be
overwritten. The database collects usage statistics and tunes the undo retention
period based on these statistics and on undo tablespace size. Provided that automatic
undo management is enabled, the database automatically tunes the undo retention
period as follows:
■ For an AUTOEXTEND undo tablespace, the database tunes the undo retention
period to be slightly longer than the longest-running query, if space allows. In
addition, when there is adequate free space, the tuned retention period does not
go below the value of the UNDO_RETENTION initialization parameter.
■ For a fixed size undo tablespace, the database tunes for the maximum possible
undo retention. This means always providing the longest possible retention
period
while avoiding out-of-space conditions and near out-of-space conditions in the
undo tablespace. The UNDO_RETENTION initialization parameter is ignored unless
retention guarantee is enabled.
■ Automatic tuning of undo retention is not supported for LOBs. The tuned
retention value for LOB columns is set to the value of the UNDO_RETENTION
parameter.
For fixed size and AUTOEXTEND undo tablespaces of equal size, depending on the
queries that you run, the tuning method used in fixed size tablespaces tends to
provide a longer retention period. This enables flashback operations to flash
back farther in time, and maximizes the amount of undo data available for
long-running queries.