The PGA is process-specific memory for individual sessions, while the SGA is shared memory for all database processes. 1. The PGA holds session variables, SQL execution memory, and cursor state, private to each user connection. 2. The SGA includes the buffer cache, redo log buffer, shared pool, large pool, Java pool, and Streams pool, shared among all processes. 3. They work together during operations such as queries, where sorting occurs in the PGA and data blocks are fetched via the SGA. 4. Memory-intensive operations may spill to temp space if PGA is undersized. 5. PGA tuning is needed for slow queries, large sorts, or multi-pass executions, while SGA tuning addresses cache contention, low hit ratios, and redo wait events. Proper management of both ensures optimal performance.

In Oracle architecture, the Program Global Area (PGA) and the System Global Area (SGA) play different but complementary roles. The main difference is that the PGA is process-specific, meaning it's private to each session or connection, while the SGA is a shared memory area used by all database processes.

What Is the PGA?

The PGA, or Program Global Area, is a memory region allocated for each individual process connecting to the Oracle database. It holds data and control information specific to a server process — things like sorting operations, session variables, and bind variable values.

• Private memory per session: Each connected user gets their own PGA.
• Managed automatically in newer versions (like 11g and later) when using automatic PGA management.
• Contains things like:
  • Session variables
  • Runtime memory for SQL execution (especially sorts and hash joins)
  • Cursor state information

So if you're running a query that needs a large sort, that operation will likely use PGA memory. If this area isn't sized properly, you might start seeing performance issues or even disk-based sorting, which is slower.

What Is the SGA?

The SGA, or System Global Area, is a shared memory pool that contains data and control information for one Oracle instance. It’s accessible by all server and background processes.

• Shared among all sessions and processes
• Includes several key components:
  • Database buffer cache (holds copies of data blocks)
  • Redo log buffer (records changes made to the database)
  • Shared pool (contains parsed SQL, PL/SQL code, dictionary cache)
  • Large pool (optional, used for backup, RMAN, etc.)
  • Java pool and Streams pool (used for Java and Oracle Streams features)

Because it’s shared, the SGA is critical for overall database performance. If it’s too small, you may see increased I/O activity or library cache contention.

How Do They Work Together?

Even though they serve different purposes, the PGA and SGA often work together during database operations. For example:

• When a user runs a query, some of the processing happens in the PGA (such as sorting), and the data blocks are fetched from the SGA (or disk via the buffer cache).
• Memory-intensive operations like hash joins or sorts may spill over into temporary tablespace if the PGA is undersized.
• In environments with many concurrent users, the total PGA usage can add up quickly, so tuning is important to avoid excessive memory consumption.

One thing to note: in Oracle versions prior to 12c, PGA memory was not automatically managed unless enabled. With Automatic Memory Management (AMM), Oracle can dynamically adjust both SGA and PGA sizes based on workload.

When to Tune PGA vs SGA

You’ll want to look at PGA tuning when:

• You’re doing large sorts or hash joins
• You see "multi-pass" executions in SQL plans (meaning more than one pass through temp space)
• Users complain about slow query performance despite having good indexes

For SGA tuning, watch for:

• Buffer cache hit ratio dropping below acceptable levels
• Library cache or row cache contention
• Frequent log switches or redo wait events

Tools like AWR reports, V$ views (V$PGASTAT, V$PROCESS, V$SESSTAT), and Enterprise Manager can help identify bottlenecks in either area.

They’re different in purpose but both crucial — PGA handles per-session needs, and SGA serves everyone collectively. Keeping an eye on how your system uses each helps maintain smooth performance.

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