Oracle Database - Physical I/O

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A physical read, I/O of table or index data places the block into the buffer cache. Then we perform a logical io to retrieve the block. Hence, most physical read are immediately followed by a logical io.

There are two major types of common physical I/Os :

  • reading the data in from datafiles. Doing I/O to the datafiles to retrieve index and table data. These operations will be followed immediately by a Logical IO to the cache.
  • direct read from TEMP. This in response to a sort area or hash area not being large enough to support the entire sort/hash in memory. The database is forced to swap out some of the data to TEMP and read it back later. These physical reads bypass the buffer cache and will not incur a logical I/O.

The physical I/O is measured by the physical reads statistic.

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