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Oracle PGA memory allocation for dedicated connections

When a dedicated connection is made to Oracle, an isolated memory region called the program Global Area is allocated in UNIX RAM memory. The PGA consists of the following components:

* Sort area – This is the largest and most important area of the PGA
* Session information – This small area contains internal address for the connection to allow the connection to communicate with Oracle.
* Cursor state – This component of the PGA contains all reentrant values for the executing connection.
* Stack space – This area contains miscellaneous control structures.

The largest component of a PGA is the sort area size, and Oracle allows you to dynamically change the sort area size at the session level.

alter session set sort_area_size=10m deferred;

When you issue this alter session command, you instruct UNIX to expand the sort area within the PGA at the time that the sort is required. To illustrate the deferred RAM memory allocation in UNIX.

Here we see that Oracle interfaces with UNIX to issue the malloc() command to provide a RAM sort area. This RAM region is only allocated after the retrieval from the database has been completed, and the memory only exists for the duration that the sort is required. This technique reduces the RAM memory demands on the UNIX server and ensures that the RAM is only available when it is needed by Oracle.

The above is an excerpt from the "Oracle UNIX Administration Handbook" by Oracle press, authored by Donald K. Burleson.


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