SGA Memory Issue in Oracle

 

🔴 Problem

A production database suddenly becomes very slow. Users report:

• Application taking too long to respond

• Queries that normally run in seconds taking minutes

• High wait events in database

The DBA checks the alert log and finds memory-related errors like:

ORA-04031: unable to allocate bytes of shared memory

This error occurs in the SGA (System Global Area).

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📌 What is SGA

SGA is the shared memory area used by Oracle to store:

• Database buffer cache

• Shared pool

• Large pool

• Java pool

• Redo log buffer

If SGA components are not sized properly, Oracle cannot allocate memory.

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🔎 Investigation Steps (DBA Approach)

1️⃣ Check SGA size

show parameter sga;

Example output:

sga_target = 4G
sga_max_size = 4G

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2️⃣ Check shared pool usage

SELECT pool, name, bytes
FROM v$sgastat
WHERE pool='shared pool'
ORDER BY bytes DESC;

This helps identify memory allocation in the shared pool.

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3️⃣ Check memory dynamic components

SELECT component,
       current_size,
       min_size,
       max_size
FROM v$sga_dynamic_components;

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4️⃣ Check for ORA-4031 details

SELECT *
FROM v$sgastat
WHERE name LIKE '%free memory%';

If free memory is very low, the shared pool is exhausted.

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🛠 Solution

Increase Shared Pool Size

ALTER SYSTEM SET shared_pool_size=800M SCOPE=BOTH;

OR increase SGA target

ALTER SYSTEM SET sga_target=6G SCOPE=BOTH;

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⚠️ Important Checks Before Increasing Memory

DBAs should verify:

• Server RAM availability

• Memory fragmentation

• Hard parsing issues

• Too many unique SQL statements

Check SQL parsing:

SELECT sql_text, executions
FROM v$sqlarea
ORDER BY executions DESC;

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📊 Root Cause (Real Production Case)

In one production environment:

• Application generated thousands of dynamic SQL queries

• No bind variables used

• Shared pool filled quickly

• Result → ORA-04031 error

Application server involved: Apache Tomcat

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✅ Final Fix Implemented

1️⃣ Increased SGA memory

SGA_TARGET = 8G

2️⃣ Increased shared pool

SHARED_POOL_SIZE = 1G

3️⃣ Developers implemented bind variables

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📘 Key Learning for DBAs

Always monitor:

• v$sga

• v$sgastat

• v$sga_dynamic_components

• v$sqlarea

Never just increase memory without identifying the root cause.