Beginning Oracle Database 11g Administration From Novice to Professional

CHAPTER 17 SQL TUNING 395
Sorts (memory) and sorts (disk) track the number of sorting operations performed
by Oracle during the course of our query. Sorting operations are performed in
memory if possible; they spill onto the disks if the data does not fit into Oracle’s
memory buffers. It is desirable for sorting to be performed in memory because
reading and writing data to and from the disks are expensive operations. The
amount of memory available to database sessions for sorting operations is controlled
by the value of .
rows processed is the number of rows of information required by the user.
The preceding explanations should make it clear that a lot of overhead is involved in
executing a query—for example, physical reads are required if data blocks are not found
in Oracle’s cache. There were no physical reads necessary when we executed our query
the first time, indicating that all the required blocks were found in the cache. We can
gauge the extent of overhead by flushing the shared pool (cache of query execution plans)
and buffer cache (cache of data blocks) and executing the query again:
The numbers rise dramatically; 1,653 recursive calls and 498 consistent gets (including
136 physical reads) are required after the flush. The number of sort operations also
jumps—from 0 to 37: