EscVector wrote:
> I'm bottlenecked on db file sequential read. What wait should I expect
> to see if the plan shows Index Fast Full Scan? Is it db file
> sequential read or scattered read since the Fast Full Scan is treated
> like a tablescan and uses multiblock reads?

Paraphrased from various sources:
Oracle issues a scattered read to read data into multiple discontinuous
memory locations. A scattered read is usually a multiblock read. It
can occur for a fast full scan of an index in addition to the more
common full table scan. Db file scattered read wait events indicate
that a full scan is occurring. When performing a full scan into the
buffer cache, the blocks read are read into memory locations that are
not physically adjacent to each other.

A sequential read is a single-block read, where a user process is
reading a buffer into the SGA buffer cache and waiting for a physical
I/O. Single block I/Os are usually the result of index scans. Rarely,
full table scans could get truncated to a single block call due to
extent boundaries, or buffers already present in the buffer cache.
------------------------------

An index fast full scan should appear as db file scattered read. The
second paragraph above seems to indicate that full table scans can wait
on a db file sequential read event, if most of the required blocks are
already in the buffer cache, and no contiguous blocks are required. I
suspect that the same is true for index fast full scans.

Charles Hooper
PC Support Specialist
K&M Machine-Fabricating, Inc.