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This article will explain in detail how to analyze the impact of Oracle high water level (HWM) on select performance. The content of the article is of high quality, so the editor shares it for you as a reference. I hope you will have some understanding of the relevant knowledge after reading this article.

I don't know if you have encountered any feedback from the app to you, "there is no data in my table, but why is the access so slow? ask DBA to help you analyze it." then you look at the table, and then you select it, and you find it is really slow. Then you ask the business staff that they delete 10 million rows of data before checking, and then the query is slow.

In fact, this is the problem, because it is a delete operation, did not reduce the location of HWM, here introduced the oracle access table data way: oracle will access all the blocks below the HWM, if your HWM position is very high, even if your HWM under all the blocks there are few pieces of data, but select will not be fast, this is the problem of slow access.

Now that a problem has been found, how to solve it?

We said earlier that the truncate operation can clear the high water level. So if you want to delete data in large quantities, try to use truncate if you can.

We know that after 11g, our segment space management is generally automatic segment space management, that is, automatic segment space management, and then assm introduces the concept of low HWM. When the data is inserted into the new data block, it does not format the blocks without being used. Then the concept of low HWM is to mark these blocks that have already been formatted, so low HWM must

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