DB File Sequential and Scattered Reads
Both “db file sequential read” and “db file scattered read” events signify time waited for I/O read requests to complete. Time is reported in 100’s of a second for Oracle 8i releases and below, and 1000’s of a second for Oracle 9i 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.
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.
The index full scan typically uses db file sequential reads to get data from disk, although newer versions of Oracle can do db file parallel reads which are read requests for multiple Oracle blocks that are not adjacent blocks in the Oracle data files.
The index fast full scan typically uses db file scattered reads to get data from disk and the indexes are then subject to all the details of the special treatment for tablescans.
Then it will usually be a read from a SORT (TEMPORARY) Segment as multiblock reads for table scansusually show up as waiting on “db file scattered read”.
Sometimes delayed block cleanout also cause db file scattered read event.
Related wait events:
direct path read
buffer busy waits
read by another session
Just for the record
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