chap15_11.html
来自「Oracle资料大集合」· HTML 代码 · 共 305 行
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I know that Oracle performs some sorts on disk and some sorts in memory and that memory-based sorts are faster than disk sorts. How can I tell if Oracle is performing too many disk-based sorts? If this is the case, how can I improve the situation?<p><B>Technique</B><p>When a user connects to the Oracle server and creates a session, Oracle creates an area in memory called the Process Global Area or Program Global Area (PGA). Part of this area is dedicated to user-requested sorts. Index creation, ORDER BY and DISTINCT clauses, and some joins all cause Oracle to undertake sorting operations.<p>If the sort area in the PGA is large enough then Oracle performs the sort in memory. Otherwise, Oracle splits the data to be sorted into smaller pieces, sorts them, writes them to temporary segments on disk, and finally reassembles them to complete the sort. This is one way that the Oracle server may use disk space, as opposed to memory, to complete a sort.<p>Oracle may also utilize disk for sorts, even when the entire sort could occur in memory. The sort could begin in memory, for example, but then Oracle may attempt to free some PGA memory to reference some data that is not part of the sort area. In this case, Oracle will write a temporary disk segment to contain some of the sorted data.<p>The UTLESTAT report contains a section summarizing sort performance; it appears in Listing 15.14.<p>Listing 15.14 An excerpt from the FILE I/O section of the UTLESTAT report<p><code><b>SVRMGR> Rem The total is the total value of the statistic between the time<p>SVRMGR> Rem bstat was run and the time estat was run. Note that the estat<p>SVRMGR> Rem script logs on as 搃nternal⌨️ 快捷键说明
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