Sunday Mar 17, 2013

关于10053 trace中的UNCOMPBKTS和ENDPTVALS

当Histogram直方图类型为frequency histograms( Histogram: Freq)时UncompBkts 等于表上的行数, 而EndPtVals 等于bucket总数,或者说NDV,因为frequency histograms中 NDV=number of buckets 

当直方图类型为height balanced histograms (Histogram: HtBal) UncompBkts  等于bucket的数目(其实也等于10053 trace中#Bkts的数目),而EndPtVals 等于已经被压缩的Histogram的大小,其实是等于: select count(*) from dba_tab_histograms where table_name=’MACLEANLIU’ and column_name=’MACLEANLIU’的实际总和。  通过这2个值对比,可以了解到popular值的多少以及数据的倾斜度, 是有多个大量重复的值(popular value)还是仅有一个巨大的重复值。

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author's avatar The Maclean Liu
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