direct path read
During Direct Path operations the data is asynchronously read from the database files. At some stage 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).
Detailed Description​
You will see direct path read and direct path read temp waits when:
Sorts are too large for memory and have been written to disk.
When parallel execution servers are scanning for data
When the server process is processing buffers faster than the I/O system can return them.
Direct path reads load data from disk directly into your processes private memory - the Program Global Area (PGA) rather than the shared memory normally associated with database reads. Asynchronous I/O will be used if possible.
How to reduce this wait​
Use the tuning script from oracle-base.com and look in the 'Disk Sorts' section to see if you have excessive sorts to disk occurring. Tune the SQL to reduce sorting, and consider adjusting the PGA_AGGREGATE_SIZE parameter to allow more sorting in memory.
If you have a decision support system running parallel queries, this will be the mechanism used to return data.
Additional Links​
Oracle Performance Tuning Guide - direct path read and direct path read temp
Oracle Reference - PGA_AGGREGATE_TARGET
Tuning script from oracle-base.com
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