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Expert Oracle Database Architecture, Third Edition by Darl Kuhn, Thomas Kyte

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The HASHED implementation simply converted the OBJECT_ID passed into the query into a FILE/BLOCK to be read and read it—no index. The HEAP table, however, had to do two I/Os on the index for each row. The cr=2 in the TKPROF Row Source Operation line shows us exactly how many consistent reads were done against the index. Each time I looked up OBJECT_ID = :B1, Oracle had to get the root block of the index and then find the leaf block containing the location of that row. Then, I had to take the leaf block information, which included the ROWID of that row, and access that row in the table for a third I/O. The HEAP table did three times the I/O of the HASHED implementation.

The points of interest here are as follows:

  • The hash cluster did significantly ...

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