Oracle Database Memory Monitoring Guide

Introduction

Database memory management is crucial for maintaining optimal performance in Oracle databases. Inefficient memory usage can lead to slower query execution, increased I/O operations, and poor application response times. This comprehensive guide provides database administrators and developers with practical tools and techniques for monitoring and optimizing Oracle database memory components.

Why Memory Monitoring Matters

• Performance Impact: Memory issues account for approximately 40% of database performance problems
• Resource Optimization: Proper monitoring helps in right-sizing memory allocation
• Cost Efficiency: Optimal memory usage reduces hardware requirements and cloud costs
• Proactive Management: Early detection of memory issues prevents production incidents

1. Shared Pool Memory Usage

What it means: Shows how efficiently SQL statements are being reused and percentage of free memory available.

WITH shared_pool_total AS (
    SELECT SUM(bytes) total_bytes
    FROM v$sgastat 
    WHERE pool = 'shared pool'
)
SELECT s.pool, 
       s.name, 
       ROUND(s.bytes/1024/1024, 2) MB,
       ROUND((s.bytes/t.total_bytes)*100, 2) AS percentage
FROM v$sgastat s, shared_pool_total t
WHERE s.pool = 'shared pool'
AND s.bytes > 1024*1024  -- Show components using >1MB
ORDER BY s.bytes DESC;

Interpretation:

• “free memory” percentage <5% → Critical, increase shared_pool_size
• “free memory” percentage >20% → Consider reducing shared_pool_size
• “library cache” >30% → Good SQL reuse
• “dictionary cache” >15% → Efficient data dictionary access

2. Buffer Cache Efficiency

What it means: Shows buffer cache hit ratio and physical vs. logical read percentages.

WITH stats AS (
    SELECT name, value
    FROM v$sysstat
    WHERE name IN (
        'physical reads',
        'db block gets',
        'consistent gets'
    )
)
SELECT 
    ROUND((SUM(CASE WHEN name IN ('db block gets', 'consistent gets') 
                    THEN value ELSE 0 END) - 
           SUM(CASE WHEN name = 'physical reads' 
                    THEN value ELSE 0 END)) /
          NULLIF(SUM(CASE WHEN name IN ('db block gets', 'consistent gets') 
                         THEN value ELSE 0 END), 0) * 100, 2) 
    AS buffer_cache_hit_ratio,
    SUM(CASE WHEN name = 'physical reads' THEN value ELSE 0 END) AS physical_reads,
    SUM(CASE WHEN name IN ('db block gets', 'consistent gets') 
             THEN value ELSE 0 END) AS logical_reads
FROM stats;

Interpretation:

• Buffer Cache Hit Ratio <95% → Consider increasing db_cache_size
• High physical reads relative to logical reads → Review I/O intensive queries
• Track trend of physical:logical read ratio over time

3. User Memory Usage Summary

What it means: Shows PGA memory usage aggregated by user, helping identify memory-intensive applications or user sessions.

WITH total_pga AS (
    SELECT SUM(pga_used_mem) total_used,
           SUM(pga_alloc_mem) total_alloc
    FROM v$process
)
SELECT 
    s.username,
    SUM(ROUND(p.pga_used_mem/1024/1024,2)) used_mb,
    SUM(ROUND(p.pga_alloc_mem/1024/1024,2)) alloc_mb,
    SUM(ROUND(p.pga_max_mem/1024/1024,2)) max_mb,
    ROUND((SUM(p.pga_used_mem)/t.total_used)*100, 2) pct_of_total_used,
    ROUND((SUM(p.pga_alloc_mem)/t.total_alloc)*100, 2) pct_of_total_alloc
FROM v$session s
JOIN v$process p ON s.paddr = p.addr
CROSS JOIN total_pga t
WHERE s.type = 'USER'
AND s.username IS NOT NULL
GROUP BY s.username, t.total_used, t.total_alloc
ORDER BY used_mb DESC;

Interpretation:

• Users consuming >20% of total PGA → Review their workload
• Large gap between used_mb and alloc_mb → Memory inefficiency
• High max_mb relative to used_mb → Spiky memory usage pattern

4. Sort Operations Analysis

What it means: Shows the efficiency of sort operations and whether they’re happening in memory vs. disk.

WITH sort_stats AS (
    SELECT name, value
    FROM v$sysstat
    WHERE name IN (
        'sorts (memory)',
        'sorts (disk)',
        'sorts (rows)'
    )
)
SELECT 
    name,
    value,
    ROUND(value * 100.0 / 
          NULLIF(SUM(CASE WHEN name IN ('sorts (memory)', 'sorts (disk)') 
                         THEN value ELSE 0 END) 
                OVER (), 0), 2) as percentage
FROM sort_stats;

Interpretation:

• Memory sorts should be >95% of total sorts
• Disk sorts >5% → Increase PGA or review sort-intensive queries
• High total sorts → Review indexing strategy and SQL that causes sorts

Note: All these queries show USED memory rather than just allocated. For example:

• Shared pool query shows actual usage of each component
• Buffer cache query shows actual reads vs. potential reads
• PGA query explicitly shows used vs. allocated memory
• Sort operations show actual sorts performed

The main difference between used and allocated memory:

• Used: Actually being used by database processes
• Allocated: Reserved but may not be actively used
• The gap between these two indicates memory efficiency

Oracle Memory Architecture Overview (official document) which help you Oracle Database Memory Monitoring can be found here.

• Oracle Database Memory Management
• Memory Management Guide

Would you like me to add any other specific memory metrics or modify any of these queries further?