Oracle Database SGA Breakdown Script for Performance Monitoring using v$sgastat

calendar Aug 20, 2025 / · 3 min read · oracle database dba performance sql tuning scripts memory management v$sgastat  ·
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Oracle Database SGA Breakdown Script for Performance Monitoring

Monitoring memory usage in an Oracle Database is one of the most important tasks for a DBA. The System Global Area (SGA) plays a crucial role in the performance of the database, as it stores data and control information used by Oracle processes.

In this post, we will explore an SQL script that provides a breakdown of SGA memory pools using the v$sgastat dynamic performance view. We'll go over its purpose, SQL breakdown, key points, and insights for DBAs.

Purpose of the Script

The purpose of this script is to give DBAs visibility into how Oracle allocates memory across the Shared Pool, Large Pool, Java Pool, Streams Pool, and Buffer Cache. By analyzing this breakdown, DBAs can:

  • Understand memory consumption patterns.
  • Identify memory fragmentation.
  • Optimize performance by adjusting SGA_TARGET or specific pool sizes.
  • Troubleshoot "ORA-04031: unable to allocate memory in SGA" errors.

The Script

 1set lines 100 pages 999
 2col bytes format 999,999,999
 3compute sum of bytes on pool
 4break on pool skip 1
 5select pool
 6     , name
 7     , bytes
 8from v$sgastat
 9order by pool, name
10/

Script Breakdown

  • set lines 100 pages 999 Improves readability by formatting output across 100 characters per line and setting a large page size for continuous results.
  • col bytes format 999,999,999 Formats the bytes column to display values with commas, making large numbers easier to interpret.
  • compute sum of bytes on pool Automatically computes and displays the total memory used for each individual SGA pool.
  • break on pool skip 1 Ensures that results are grouped by "pool" and adds a line break for clarity.
  • select pool, name, bytes from v$sgastat order by pool, name Fetches all memory allocation statistics from v$sgastat, categorized by memory pool and component (name).

Key Points DBAs Should Know

  • Each pool in Oracle SGA has a specific purpose:
    • Shared Pool: Caching SQL statements, execution plans, dictionary metadata.
    • Large Pool: Memory for large I/O operations like RMAN backup and parallel queries.
    • Java Pool: Used for JVM memory inside Oracle Database.
    • Streams Pool: Stores memory for Oracle Streams and GoldenGate replication.
    • Buffer Cache: Holds frequently accessed data blocks to reduce disk I/O.
  • Monitoring v$sgastat helps identify inefficient memory allocation.
  • If you see a lot of "free memory" in some pools but shortages in others, you may need to rebalance sizing.
  • Oracle’s Automatic Shared Memory Management (ASMM) dynamically manages most of these values when enabled, but analysis of v$sgastat still provides great diagnostic insight.

Practical Insights

  • Run this script during peak load to identify constraints in memory pools.
  • Compare differences before and after query execution to see memory usage impact.
  • Some large pools may show 'free memory' indicating underutilization—this is often fine but should still be monitored.
  • Persistent shortages often require adjusting init.ora parameters like SGA_TARGET, SGA_MAX_SIZE, or specific pool sizes (SHARED_POOL_SIZE, LARGE_POOL_SIZE, etc.).

Conclusion

This Oracle SGA breakdown script provides DBAs with a quick yet powerful way to understand memory allocations within the database. By leveraging v$sgastat, you can tune Oracle performance, prevent errors, and optimize resource utilization.

References

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