Sometimes related to sorting operations, check to see if occurring primarily in temp tablespaces. During Direct Path operations the data is asynchronously read from the database files. At some point the session needs to make sure that all outstanding asynchronous I/O have been completed to disk. This can also happen if, during a direct read, no more slots are available to store outstanding load requests (a load request could consist of multiple I/Os). Direct path reads are generally used by Oracle when reading directly into PGA memory (as opposed to into the buffer cache). If asynchronous I/O is supported (and in use) then Oracle can submit I/O requests and continue processing. It can then pick up the results of the I/O request later and will wait on "direct path read" until the required I/O completes.
Solutions
1) Reading from temp tablespaces
If the database parameter workarea_size_policy = AUTO:
- Increase the database parameter 'pga_aggregate_target'
Otherwise:
- Increase the database parameter 'sort_area_size'.
Another possible solution is to cache the temp datafile at the O/S or Storage system level.
2) Reading application data
a. Ensure database parameter, DISK_ASYNC_IO is TRUE
b. Ensure no disks are I/O bound.
c. Ensure the OS asynchronous I/O is configured correctly.