Oracle internals, debugging and undocumented features
Direct path reads and serial table scans in 11g
July 17, 2009Posted by on
To answer comment by Frits about Doug Burns observation of using full table scans that resulted in direct path reads.
Here is explanation from Oracle itself:
There have been changes in 11g in the heuristics to choose between direct path reads or reads through buffer cache for serial table scans.
In 10g, serial table scans for “large” tables used to go through cache (by default) which is not the case anymore. In 11g, this decision to read via direct path or through cache is based on the size of the table, buffer cache size and various other stats.
Direct path reads are faster than scattered reads and have less impact on other processes because they avoid latches.
References: High ‘direct path read’ waits in 11g Note: 793845.1