ORACLE推導參數Derived Parameter介紹

Oracle的推導參數（Derived Parameters）其實是初始化參數的一種。推導參數值通常來自于其它參數的運算，依賴其它參數計算得出。官方文檔關于推導參數（Derived Parameters）的概念如下：

<b>Derived Parameters</b>

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Some initialization parameters are derived, meaning that their values are calculated from the values of other parameters. Normally, you should not alter values for derived parameters, but if you do, then the value you specify will override the calculated value.

For example, the default value of the SESSIONS parameter is derived from the value of the PROCESSES parameter. If the value of PROCESSES changes, then the default value of SESSIONS changes as well, unless you override it with a specified value.

很奇怪的是官方資料關于推導參數（Derived Parameters）的介紹非常少，幾乎就是那麼一點，無法從v$parameter等系統視圖擷取那些是推導參數（Derived Parameters），查了一些資料似乎還有下面一些參數是推導參數.

· <b>_enqueue_hash_chains-</b> The default value is derived from processesparameter.

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· <b>db_block_checkpoint_batch </b>- This parameter specifies the number of blocks that the DBWR writes in one batch when performing a checkpoint. Setting this value too high causes the system to flood the I/O devices during the checkpoint, severely degrades performance, and increases response times--maybe to unacceptable levels.

· <b>enqueue_resources</b><b> - </b>This parameter specifies the number of resources that can be locked by the lock manager. The default value is derived fromprocesses and is usually sufficient.

· <b>nls_currency</b><b> - </b>This parameter is derived from nls_territory, and specifies the string to use as the local currency symbol for the L number format element.

· <b>nls_date_format</b><b> - </b>This parameter is derived from nls_territory and definesthe default date format to use with the to_char and to_date functions. The value of this parameter is any valid date format mask.

· <b>nls_iso_currency</b><b> </b>- Derived from nls_territory, this parameter defines the string to use as the international currency symbol for the C number format element.

· <b>nls_numeric_characters</b><b> </b>- This is derived from nls_territory, and defines the characters to be used as the group separator and decimal.

· <b>nls_sort</b><b> </b>- Derived from nls_language, this<b> </b>parameter is set to BINARY, the collating sequence for ORDER BY is based on the numeric values of the characters. A linguistic sort decides the order based on the defined linguistic sort. A binary sort is much more efficient and uses much less overhead.

· <b>sessions </b>- This parameter specifies the total number of user and system sessions, and is set to 1.1 times the value of the processes parameter.

同時修改參數sessions和processes，然後重新開機資料庫，然後檢查參數processes與sessions的關系。

<a href="http://images2015.cnblogs.com/blog/73542/201610/73542-20161030234640375-485297146.png"></a>

如上所示，processes與sessions的關系已經不成立了：sessions=(1.1 * processes) + 5（Oracle 10g）。主要還是因為推導參數session設定後，覆寫了推導值。這個參數值已經寫入了參數檔案spfile或pfile當中。

參考資料：

<a href="http://www.dba-oracle.com/t_derived_parameters.htm">http://www.dba-oracle.com/t_derived_parameters.htm</a>