03.21.06

pctversion in lob segment

Posted in Oracle at 8:46 下午 由 wanghai

在oracle的官方文档上有这么一段话来解释pctversion

PCTVERSION integer
Specify the maximum percentage of overall LOB storage space used for maintaining old versions of the LOB. The default value is 10, meaning that older versions of the LOB data are not overwritten until they consume 10% of the overall LOB storage space.
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03.15.06

parallel query in partition table

Posted in Oracle at 10:46 下午 由 wanghai

上一篇文章里面提到parallel query相关的checkpoint,也了解了direct read会对系统性能带来的潜在影响，这次让我们看一下parallel query在partition table上的表现。
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DBWR parallel query checkpoint buffers written

Posted in Oracle at 2:14 上午 由 wanghai

DBWR parallel query checkpoint buffers written是oracle的一种特殊的checkpoint的产物，这种checkpoint在oracle8 以前称作Extent-based Checkpoint，当发生parallel query时oracle必须做checkpoint把脏数据写入磁盘，因为parallel query走的时direct read，直接从文件读入pga，如果有脏数据没有被写回磁盘，那么读出来的结果将会不一致。
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