What is the difference between DB file sequential and scattered reads?

hi motaz well this is the amswer for ur question A db file sequential read is an event that shows a wait for a foreground process while doing a sequential read from the database.
This is an o/s operation, most commonly used for single block reads.
Single block reads are mostly commonly seen for index block access or table block access by a rowid (Eg: to access a table block after an index entry has been seen) This can also be a multiblock read.
Then it will usually be a read from a SORT (TEMPORARY) segment as multiblock reads for table scans (and index fast full scans) usually show up as waiting on "db file scattered read" A db file scattered read is the same type of event as "db file sequential read", except that Oracle will read multiple data blocks.
Multi-block reads are typically used on full table scans.
The name "scattered read" may seem misleading but it refers to the fact that multiple blocks are read into DB block buffers that are 'scattered' throughout memory.

Both "db file sequential read" and "db file scattered read" events signify time waited for I/O read requests to complete. Time is reported in100's of a second for Oracle8i releases and below, and1000's of a second for Oracle9i and above. Most people confuse these events with each other as they think of how data is read from disk. Instead they should think of how data is read into the SGA buffer cache or user PGA memory.

 

db file sequential read:

 

A sequential read operation reads data into contiguous memory (usually a single-block read with p3=1, but can be multiple blocks). Single block I/Os are usually the result of using indexes. This event is also used for rebuilding the controlfile and reading datafile headers (P2=1). In general, this event is indicative of disk contention on index reads.

 

db file scattered read:

 

Similar to db file sequential reads, except that the session is reading multiple data blocks and scatters them into different discontinuous buffers in the SGA. This statistic is NORMALLY indicating disk contention on full table scans. Rarely, data from full table scans could be fitted into a contiguous buffer area, these waits would then show up as sequential reads instead of scattered reads. However, scattered read will never read just one block (p3 is always >=2).