🔴 Problem
Users complain that:
-
Queries are running very slowly
-
Some sessions are consuming very high memory
-
Server memory usage is extremely high
-
Database performance suddenly drops
During investigation, the DBA finds excessive PGA memory usage.
Sometimes errors like:
ORA-04036: PGA memory used by the instance exceeds PGA_AGGREGATE_LIMIT
📌 What is PGA
PGA (Program Global Area) is private memory allocated for each server process.
It is used for:
-
Sorting operations
-
Hash joins
-
Bitmap merge
-
Session memory
-
SQL work areas
Unlike SGA, PGA is not shared between sessions.
🔎 Investigation Steps
1️⃣ Check PGA parameters
show parameter pga;
Example output:
pga_aggregate_target = 2G
pga_aggregate_limit = 4G
2️⃣ Check PGA usage
SELECT name, value
FROM v$pgastat;
Important metrics:
-
total PGA allocated
-
maximum PGA allocated
-
cache hit percentage
3️⃣ Identify sessions consuming high PGA
SELECT
s.sid,
s.serial#,
p.pga_used_mem,
p.pga_alloc_mem
FROM v$session s, v$process p
WHERE s.paddr = p.addr
ORDER BY p.pga_used_mem DESC;
This shows which session is using the most PGA.
4️⃣ Check SQL causing large sorts
SELECT sql_id,
executions,
disk_reads,
buffer_gets
FROM v$sql
ORDER BY disk_reads DESC;
Large sorts or hash joins often consume high PGA.
🛠 Solution
Increase PGA target
ALTER SYSTEM SET pga_aggregate_target=4G SCOPE=BOTH;
If limit is too small:
ALTER SYSTEM SET pga_aggregate_limit=6G SCOPE=BOTH;
⚠️ Important DBA Checks
Before increasing PGA:
-
Verify server RAM availability
-
Identify runaway queries
-
Check parallel queries
-
Check large sorting operations
Example check:
SELECT *
FROM v$sql_workarea_active;
This shows active work areas consuming PGA.
📊 Root Cause (Real Production Case)
In one production system:
-
A reporting query ran with large ORDER BY and GROUP BY
-
Query processed millions of rows
-
Sort operation moved to disk
-
PGA usage increased rapidly
Application server: Oracle WebLogic Server
✅ Final Fix Implemented
1️⃣ Increased PGA
PGA_AGGREGATE_TARGET = 6G
2️⃣ Optimized query with indexes
3️⃣ Reduced unnecessary sorting
4️⃣ Limited parallel query usage
📘 Key Learning for Oracle DBAs
Always monitor:
-
v$pgastat -
v$process -
v$sql_workarea -
v$sql_workarea_active
High PGA usage is usually caused by:
-
Large sorts
-
Hash joins
-
Parallel queries
-
Poorly optimized SQL
