“direct path read temp” and “direct path read” event are wait events.
When a session is reading buffers from disk directly into the PGA (opposed to the buffer cache in SGA), it waits on this event.
If the asynchronous I/Os is by the I/O subsystem:
Check the following VSESSION_WAIT parameter columns:
This happens in the following situations:
The file_id shows if the reads are for an object in TEMP tablespace (sorts to disk) or full table scans by parallel slaves. This is the biggest wait for large data warehouse sites. However, if the workload is not a DSS workload, then examine why this is happening.
Examine the SQL statement currently being run by the session experiencing waits to see what is causing the sorts. Query VTEMPSEG_USAGE to find the SQL statement that is generating the sort. Also query the statistics from VSESSTAT for the session to determine the size of the sort. See if it is possible to reduce the sorting by tuning the SQL statement. If WORKAREA_SIZE_POLICY is MANUAL, then consider increasing the SORT_AREA_SIZE for the system (if the sorts are not too big) or for individual processes. If WORKAREA_SIZE_POLICY is AUTO, then investigate whether to increase PGA_AGGREGATE_TARGET. See “PGA Memory Management”.
If tables are defined with a high degree of parallelism, then this could skew the optimizer to use full table scans with parallel slaves. Check the object being read into using the direct path reads. If the full table scans are a valid part of the workload, then ensure that the I/O subsystem is configured adequately for the degree of parallelism. Consider using disk striping through a material solution or software solutions such as Automatic Storage Management (ASM).
For query plans that call for a hash join, excessive I/O could result from having HASH_AREA_SIZE too small. If WORKAREA_SIZE_POLICY is MANUAL, then consider increasing the HASH_AREA_SIZE for the system or for individual processes. If WORKAREA_SIZE_POLICY is AUTO, then investigate whether to increase PGA_AGGREGATE_TARGET.