understanding PGA

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Oracle memory architecture is divided in following memory structure:-

System Global Area (SGA):- This is a large, shared memory segment that virtually all Oracle processes will access at one point or another.
Process Global Area (PGA): This is memory that is private to a single process or thread; it is not accessible from other processes/threads.
User Global Area (UGA): This is memory associated with your session. It is located either in the SGA or the PGA, depending whether you are connected to the database using a shared server (it will be in the SGA), or a dedicated server (it will be in the PGA).


PGA is the memory reserved for each user process connecting to an Oracle Database and is allocated when a process is created and deallocated when a process is terminated.


Contents of PGA:-

Private SQL Area: Contains data such as bind information and run-time memory structures. It contains Persistent Area which contains bind information and is freed only when the cursor is closed and Run time Area which is created as the first step of an execute request. This area is freed only when the statement has been executed. The number of Private SQL areas that can be allocated to a user process depends on the OPEN_CURSORS initialization parameter.

Session Memory: Consists of memory allocated to hold a session’s variable and other info related to the session.

SQL Work Areas: Used for memory intensive operations such as: Sort, Hash-join, Bitmap merge, Bitmap Create.

Automatic PGA Memory Management

Before Auto-Memory management DBA had to allocate memory to:-

SORT_AREA_SIZE: The total amount of RAM that will be used to sort information before swapping out to disk.
SORT_AREA_RETAINED_SIZE: The amount of memory that will be used to hold sorted data after the sort is complete.
HASH_AREA_SIZE: The amount of memory your server process can use to store hash tables in memory. These structures are used during a hash join, typically when joining a large set with another set. The smaller of the two sets would be hashed into memory and anything that didn’t fit in the hash area region of memory would be stored in the temporary tablespace by the join key.
To enable PGA Auto-Mem Management enable the parameter WORKAREA_SIZE_POLICY and allocate total memory to be used for this purpose to PGA_AGGREGATE_TARGET.

NOTE:- From 11gR1 You can set MEMORY_TARGET and auto-mem management for both SGA and PGA is taken care.






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