ORA-04031: Unable to Allocate Shared Memory - Root Cause & Resolution Guide
The ORA-04031: unable to allocate %s bytes of shared memory error occurs when Oracle fails to allocate a contiguous block of memory in the Shared Pool or Large Pool within the System Global Area (SGA). In 90% of production environments, this error is caused by Shared Pool memory fragmentation resulting from literal SQL queries without bind variables, rather than an actual lack of total physical memory. Immediate resolution involves purging specific heavy cursors or flushing the Shared Pool, while long-term prevention requires enforcing bind variables, configuring CURSOR_SHARING = FORCE, or tuning SHARED_POOL_RESERVED_SIZE.
Environment & Prerequisites
This troubleshooting guide applies across all supported Oracle Database releases, including Single Instance, Real Application Clusters (RAC), and E-Business Suite (EBS R12.2) environments. Executing the diagnostic queries and resolution commands requires administrative privileges and direct access to dynamic performance views.
Supported Versions: Oracle Database 11g, 12c, 18c, 19c, 21c, 23c/23ai (Linux x86-64/AIX/Solaris)
Required Roles/Privileges:
SYSDBA,ALTER SYSTEM, orEXECUTEprivileges onDBMS_SHARED_POOLTarget Dynamic Views:
V$SGASTAT,V$SQLAREA,V$SQL,V$SHARED_POOL_RESERVED,V$PROCESS,V$SESSION
Problem Description & Exact Error Stack
An ORA-04031 error terminates SQL statement execution, PL/SQL package compilation, or background process tasks when an allocation request for a contiguous memory chunk fails after scanning free lists and attempting to free unpinned memory.
Command Line Execution Log
SQL> EXECUTE dbms_stats.gather_table_stats('APPS', 'OE_ORDER_HEADERS_ALL');
BEGIN dbms_stats.gather_table_stats('APPS', 'OE_ORDER_HEADERS_ALL'); END;
*
ERROR at line 1:
ORA-04031: unable to allocate 4120 bytes of shared memory ("shared pool","unknown object","sga heap(1,0)","executor tree")
Database Alert Log Snippet
2026-10-01T14:22:05.123456+08:00
Errors in file /u01/app/oracle/diag/rdbms/prod/prod1/trace/prod1_ora_54210.trc:
ORA-04031: unable to allocate 3896 bytes of shared memory ("shared pool","SELECT /*+ ALL_ROWS */...","SQLA","tmp")
What Causes ORA-04031 in Oracle Databases?
ORA-04031 stems from contiguous memory allocation failures in the SGA, primarily driven by memory fragmentation, excessive SQL hard-parsing, or improper SGA subpool sizing. Distinguishing between memory exhaustion and memory fragmentation is critical to selecting the correct resolution path.
How to Diagnose and Resolve ORA-04031 Step-by-Step
Resolving ORA-04031 requires identifying the affected subpool, pinpointing the SQL queries causing fragmentation, and applying targeted memory purging or initialization parameter adjustments. Follow these sequential steps to diagnose and clear the condition without restarting the database.
Step 1: Check Shared Pool Free Memory and Subpool Allocations
Execute the following query against V$SGASTAT to analyze memory distribution across Shared Pool components:
SELECT
subpool,
name,
ROUND(bytes/1024/1024, 2) AS size_mb
FROM v$sgastat
WHERE pool = 'shared pool'
AND name IN ('free memory', 'SQLA', 'KGH: NO ACCESS', 'library cache', 'row cache')
ORDER BY bytes DESC;
Sample Output:
SUBPOOL NAME SIZE_MB
----------- ------------------ -------
subpool 1 SQLA 842.15
subpool 1 KGH: NO ACCESS 410.50
subpool 1 library cache 215.30
subpool 1 free memory 8.12
Note: If free memory shows available space (e.g., > 50 MB) while ORA-04031 occurs, the issue is fragmentation (lack of small contiguous free chunks) rather than total memory exhaustion.
Step 2: Identify Un-Bound Literal SQL Queries Causing Fragmentation
Run this query to locate un-bound SQL statements polluting the Library Cache with single-use cursors:
SELECT
SUBSTR(sql_text, 1, 60) AS sql_snippet,
COUNT(*) AS literal_count,
ROUND(SUM(sharable_mem)/1024/1024, 2) AS total_mem_mb
FROM v$sqlarea
WHERE executions = 1
GROUP BY SUBSTR(sql_text, 1, 60)
HAVING COUNT(*) > 50
ORDER BY COUNT(*) DESC;
Sample Output:
SQL_SNIPPET LITERAL_COUNT TOTAL_MEM_MB
------------------------------------------------------------ ------------- ------------
SELECT * FROM OE_ORDER_HEADERS_ALL WHERE HEADER_ID = 100231 4120 142.50
SELECT * FROM OE_ORDER_HEADERS_ALL WHERE HEADER_ID = 100232 3890 138.10
Step 3: Check Reserved Pool Failures (V$SHARED_POOL_RESERVED)
Oracle reserves a portion of the Shared Pool for large memory allocations (> 4,400 bytes). Check if requests are failing in this region:
SELECT
free_space,
avg_free_size,
used_space,
request_failures,
last_failure_size
FROM v$shared_pool_reserved;
Sample Output:
FREE_SPACE AVG_FREE_SIZE USED_SPACE REQUEST_FAILURES LAST_FAILURE_SIZE
---------- ------------- ---------- ---------------- -----------------
12450816 14210 54210890 142 4120
Note: If REQUEST_FAILURES > 0 and LAST_FAILURE_SIZE matches your ORA-04031 error stack, SHARED_POOL_RESERVED_SIZE must be increased.
Step 4: Immediate Emergency Remediation (Non-Disruptive Memory Release)
Option A: Purge a Specific High-Memory SQL Cursor (Preferred)
Instead of flushing the entire Shared Pool, target and remove specific memory-heavy cursors using DBMS_SHARED_POOL.PURGE:
-- Find ADDRESS and HASH_VALUE for the offending SQL_ID
SELECT address, hash_value
FROM v$sqlarea
WHERE sql_id = '8f3a9bc11z0q2';
-- Output example: ADDRESS = 00000003ABC1234, HASH_VALUE = 1234567890
-- Purge the specific cursor from memory:
EXEC DBMS_SHARED_POOL.PURGE('00000003ABC1234, 1234567890', 'C');
Option B: Flush the Shared Pool (System-Wide Reset)
If system-wide fragmentation blocks database operations, clear all unpinned cursors from the Shared Pool:
ALTER SYSTEM FLUSH SHARED_POOL;
Warning: Flushing the Shared Pool causes a temporary CPU spike due to subsequent hard parsing across all active application sessions.
Step 5: Apply Permanent Fixes
Fix 1: Enforce Cursor Sharing for Un-bound Applications
If application code cannot be immediately modified to use bind variables, force Oracle to substitute literals with system bind variables dynamically:
ALTER SYSTEM SET CURSOR_SHARING = FORCE SCOPE=BOTH;
Fix 2: Increase SHARED_POOL_RESERVED_SIZE and SHARED_POOL_SIZE
Increase the reserved area allocation to prevent large block allocation failures:
-- Set Reserved Pool to 15% of Shared Pool
ALTER SYSTEM SET SHARED_POOL_RESERVED_SIZE = 256M SCOPE=BOTH;
-- Expand total Shared Pool size
ALTER SYSTEM SET SHARED_POOL_SIZE = 2G SCOPE=BOTH;
How to Verify the ORA-04031 Fix?
Verification requires confirming that memory allocation requests succeed without raising errors and that V$SHARED_POOL_RESERVED stops accumulating failure counts.
Query V$SHARED_POOL_RESERVED to verify zero new request failures following the fix:
SELECT
request_failures,
request_misses,
free_space
FROM v$shared_pool_reserved;
Sample Output:
REQUEST_FAILURES REQUEST_MISSES FREE_SPACE
---------------- -------------- ----------
0 0 42108900
Verify that contiguous free memory is available in the target subpools:
SELECT
subpool,
bytes/1024/1024 AS free_mb
FROM v$sgastat
WHERE pool = 'shared pool'
AND name = 'free memory';
Sample Output:
SUBPOOL FREE_MB
---------- -------
subpool 1 184.50
subpool 2 192.10
Prevention Best Practices & Sizing Formulas
Preventing future ORA-04031 occurrences relies on proactive memory sizing, enforcing bind variable standards in development, and pinning essential PL/SQL objects into memory during startup.
Reserved Pool Sizing Formula
Set SHARED_POOL_RESERVED_SIZE between 10% and 15% of the total SHARED_POOL_SIZE:
Pin Critical Packages into Memory
Keep core application packages pinned in the Shared Pool to prevent them from aging out and causing memory fragmentation upon re-allocation:
-- Pin heavy packages on database startup
EXEC DBMS_SHARED_POOL.KEEP('APPS.OE_ORDER_GRP', 'P');
EXEC DBMS_SHARED_POOL.KEEP('DBMS_STANDARD', 'P');
