How to Check Memory Usage in Oracle

How do I check SGA and PGA memory usage in Oracle?

The MEMORY_USAGE command inspects Oracle SGA, PGA, and object-level memory consumption to troubleshoot performance.

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Description

What does “memory usage” mean in Oracle?

Oracle splits RAM into the System Global Area (SGA), Program Global Area (PGA), and optional In-Memory Column Store. Monitoring each area prevents out-of-memory errors and keeps queries fast.

Which views expose memory usage?

Use V$SGAINFO, V$PGASTAT, V$MEMORY_DYNAMIC_COMPONENTS, and DBA_SEGMENTS. These dynamic views update continuously and require SELECT_CATALOG_ROLE.

How do I check total SGA size?

Run SELECT * FROM V$SGAINFO;. Columns CURRENT_SIZE and RESIZED show live values in bytes.

How do I check PGA usage per session?

Query V$PROCESS or V$SESSTAT joined with V$STATNAME where STATISTIC# = ‘session pga memory’.

How to detect memory-heavy tables?

Combine DBA_SEGMENTS with user tables. Large segments often trigger higher buffer cache usage.

Example: Which “Products” rows occupy most buffer cache?

SELECT p.id, p.name, COUNT(*) buffers FROM products p JOIN v$bh b ON b.objd = p.object_id GROUP BY p.id, p.name ORDER BY buffers DESC FETCH FIRST 5 ROWS ONLY;

Best practices for memory monitoring

Enable Automatic Memory Management (AMM), collect baselines in AWR, and alert on sudden 20% increases.

Common mistakes

Relying on OS tools instead of Oracle views misses shared memory. Ignoring PGA usage causes ORA-04030.

Need a quick snapshot?

Create a script that returns key columns from V$SGAINFO and V$PGASTAT every hour; store in a history table for trend analysis.

Why How to Check Memory Usage in Oracle is important

How to Check Memory Usage in Oracle Example Usage


-- Find top 3 sessions by PGA currently allocated while large 'Orders' query runs
SELECT s.sid,
       s.username,
       pga.value/1024/1024   AS pga_mb,
       sql.sql_text
FROM   v$sesstat  st
JOIN   v$statname sn  ON sn.statistic# = st.statistic#
JOIN   v$session   s  ON s.sid        = st.sid
JOIN   v$sqlarea  sql ON sql.sql_id   = s.sql_id
WHERE  sn.name = 'session pga memory'
ORDER  BY pga_mb DESC
FETCH  FIRST 3 ROWS ONLY;

How to Check Memory Usage in Oracle Syntax


-- SGA details
SELECT name, current_size/1024/1024 AS mb
FROM   v$sgainfo
ORDER  BY current_size DESC;

-- PGA aggregate statistics
SELECT name, value
FROM   v$pgastat
WHERE  name IN ('total PGA inuse', 'total PGA allocated');

-- Memory by component (11g+)
SELECT component, current_size/1024/1024 AS mb, min_size, user_specified_size
FROM   v$memory_dynamic_components;

-- Object (segment) memory footprint – ecommerce example
SELECT owner, segment_name, bytes/1024/1024 AS mb
FROM   dba_segments
WHERE  segment_name IN ('CUSTOMERS', 'ORDERS', 'PRODUCTS', 'ORDERITEMS')
ORDER  BY mb DESC;

Common Mistakes

Using operating-system tools like top or ps to estimate Oracle memory. These ignore shared memory and double-count resident sets. Always query V$ views for accurate numbers.
Monitoring only SGA and forgetting PGA. High PGA per session triggers ORA-04030 errors. Track V$PGASTAT and set PGA_AGGREGATE_LIMIT to cap runaway usage.