IO in Oracle

There are two ways in which Oracle reads blocks from disk: Either as single block reads or as multi block sequential reads.

Usually, a single block read results when an index points into a block. While the process is waiting for a single block, wait event db file sequential read is posted.

Single read blocks are usually cached in the buffer cache.

Multi block reads usually occur for full table scans. While the process is waiting for those blocks, wait event db file scattered read is posted.

Oracle maintains a statistics about physical reads.

Logical Read is the sum of consistant gets and db block gets (aka current get).

Yet to be finished.

Thanks to Murthy Parthasarathi and Guy Lambregts who each spotted an error on this page.