ORA-04031 Unable to Allocate Memory - Shared Pool Solutions

ORA-04031: Unable to Allocate Memory in Shared Pool

Error Overview

Error Text: ORA-04031: unable to allocate string bytes of shared memory ("string","string","string","string")

This error occurs when Oracle cannot allocate enough contiguous memory in the shared pool. It’s one of the most common memory-related errors and can severely impact database operations, preventing new connections and causing application failures.

Understanding the Error

Error Components

ORA-04031: unable to allocate 4096 bytes of shared memory ("shared pool","SELECT /*+ RULE */ ...","sql area","kglh0^23d1b3c8")
                             ^^^^                          ^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^     ^^^^^^^^
                             Size needed                   Pool name     SQL causing issue    Heap name

Common Memory Areas

• shared pool - General shared pool memory
• large pool - Used for parallel operations, RMAN
• java pool - Java stored procedures
• streams pool - Oracle Streams

Root Causes

1. Shared Pool Fragmentation

• Many hard parses creating small memory chunks
• Lack of bind variables causing unique SQL statements
• Memory leaks from bugs

2. Undersized Shared Pool

• Insufficient memory for workload
• Growth in concurrent users
• New applications added

3. Poor Application Design

• Not using bind variables
• Dynamic SQL generation
• Excessive hard parsing

4. Memory Leaks

• Unpinned large objects
• Cursor leaks
• Bug-related memory growth

Diagnostic Queries

Check Current Memory Status

-- Shared pool summary
SELECT pool, name, bytes/1024/1024 MB
FROM v$sgastat
WHERE pool = 'shared pool'
ORDER BY bytes DESC;

-- Free memory by pool
SELECT pool, ROUND(SUM(bytes)/1024/1024,2) as free_mb
FROM v$sgastat
WHERE name = 'free memory'
GROUP BY pool;

-- Shared pool fragmentation
SELECT bucket,
       COUNT(*) free_chunks,
       MAX(bytes)/1024 max_kb,
       SUM(bytes)/1024/1024 total_mb
FROM v$sgastat
WHERE pool = 'shared pool'
  AND name = 'free memory'
GROUP BY bucket
ORDER BY bucket;

-- Memory allocation failures
SELECT request_failures, last_failure_size
FROM v$shared_pool_reserved;

Identify Memory Consumers

-- Top memory consumers in shared pool
SELECT * FROM (
  SELECT owner, name, type, sharable_mem/1024/1024 mb
  FROM v$db_object_cache
  WHERE sharable_mem > 1048576  -- > 1MB
  ORDER BY sharable_mem DESC
) WHERE ROWNUM <= 20;

-- SQL using most memory
SELECT * FROM (
  SELECT sql_id,
         substr(sql_text,1,50) sql_text,
         sharable_mem/1024/1024 mb,
         version_count,
         loads,
         executions
  FROM v$sqlarea
  WHERE sharable_mem > 1048576
  ORDER BY sharable_mem DESC
) WHERE ROWNUM <= 20;

-- Check for cursor leaks
SELECT username,
       COUNT(*) open_cursors,
       MAX(cursor_count) max_cursors
FROM (
  SELECT s.username, s.sid,
         (SELECT COUNT(*) FROM v$open_cursor o
          WHERE o.sid = s.sid) cursor_count
  FROM v$session s
  WHERE username IS NOT NULL
)
GROUP BY username
ORDER BY open_cursors DESC;

Immediate Solutions

1. Flush Shared Pool (Temporary Fix)

-- Flush shared pool to free memory
ALTER SYSTEM FLUSH SHARED_POOL;

-- Note: This invalidates all SQL and requires reparsing
-- Only use during maintenance windows or emergencies

2. Increase Shared Pool Size

-- Check current size
SHOW PARAMETER shared_pool_size;
SHOW PARAMETER sga_target;

-- For manual memory management
ALTER SYSTEM SET shared_pool_size = 2G SCOPE=BOTH;

-- For automatic memory management
ALTER SYSTEM SET sga_target = 10G SCOPE=BOTH;
-- Let Oracle auto-tune shared pool within SGA

-- For AMM (11g+)
ALTER SYSTEM SET memory_target = 12G SCOPE=BOTH;

3. Configure Reserved Pool

-- Set reserved pool (5-10% of shared pool)
ALTER SYSTEM SET shared_pool_reserved_size = 200M SCOPE=BOTH;

-- Check reserved pool usage
SELECT request_misses, request_failures,
       free_space, used_space, requests
FROM v$shared_pool_reserved;

Long-Term Solutions

1. Implement Bind Variables

-- BAD: Unique SQL for each value
String sql = "SELECT * FROM emp WHERE emp_id = " + empId;

-- GOOD: Using bind variables
String sql = "SELECT * FROM emp WHERE emp_id = :1";
PreparedStatement ps = conn.prepareStatement(sql);
ps.setInt(1, empId);

-- PL/SQL example
DECLARE
  v_sql VARCHAR2(1000);
  v_emp_id NUMBER := 100;
  v_name VARCHAR2(50);
BEGIN
  -- BAD
  v_sql := 'SELECT name FROM emp WHERE emp_id = ' || v_emp_id;
  EXECUTE IMMEDIATE v_sql INTO v_name;

  -- GOOD
  v_sql := 'SELECT name FROM emp WHERE emp_id = :1';
  EXECUTE IMMEDIATE v_sql INTO v_name USING v_emp_id;
END;
/

2. Pin Frequently Used Objects

-- Pin important packages
BEGIN
  DBMS_SHARED_POOL.KEEP('SYS.STANDARD', 'P');
  DBMS_SHARED_POOL.KEEP('SYS.DBMS_STANDARD', 'P');
  DBMS_SHARED_POOL.KEEP('SYS.DBMS_OUTPUT', 'P');
  -- Add your critical packages
  DBMS_SHARED_POOL.KEEP('APP_SCHEMA.CRITICAL_PKG', 'P');
END;
/

-- View pinned objects
SELECT owner, name, type, kept
FROM v$db_object_cache
WHERE kept = 'YES'
ORDER BY owner, name;

3. Optimize Cursor Sharing

-- Force cursor sharing (use carefully)
ALTER SYSTEM SET cursor_sharing = FORCE SCOPE=BOTH;
-- Options: EXACT (default), SIMILAR (deprecated), FORCE

-- Monitor cursor sharing impact
SELECT force_matching_signature,
       COUNT(*) sql_count,
       SUM(sharable_mem)/1024/1024 total_mb
FROM v$sqlarea
WHERE force_matching_signature != 0
GROUP BY force_matching_signature
HAVING COUNT(*) > 10
ORDER BY sql_count DESC;

Memory Sizing Guidelines

Calculate Optimal Shared Pool Size

-- Estimate shared pool needs
WITH workload AS (
  SELECT
    COUNT(DISTINCT sql_id) unique_sqls,
    AVG(sharable_mem) avg_sql_size,
    SUM(sharable_mem)/1024/1024 total_sql_mb
  FROM v$sqlarea
)
SELECT
  unique_sqls,
  ROUND(avg_sql_size/1024) avg_sql_kb,
  ROUND(total_sql_mb) current_sql_mb,
  ROUND(total_sql_mb * 1.5) recommended_min_mb,
  ROUND(total_sql_mb * 2) recommended_optimal_mb
FROM workload;

-- Library cache hit ratio (should be > 95%)
SELECT
  namespace,
  ROUND(gethitratio * 100, 2) get_hit_ratio,
  ROUND(pinhitratio * 100, 2) pin_hit_ratio,
  reloads,
  invalidations
FROM v$librarycache
ORDER BY namespace;

Memory Advisor Recommendations

-- SGA advisor recommendations
SELECT sga_size,
       sga_size_factor,
       estd_db_time,
       estd_db_time_factor
FROM v$sga_target_advice
ORDER BY sga_size;

-- Shared pool advisor
SELECT shared_pool_size_for_estimate,
       shared_pool_size_factor,
       estd_lc_time_saved,
       estd_lc_memory_object_hits
FROM v$shared_pool_advice
ORDER BY shared_pool_size_for_estimate;

Monitoring and Prevention

Create Monitoring Framework

-- Table to track ORA-04031 occurrences
CREATE TABLE dba_04031_history (
  error_date TIMESTAMP DEFAULT SYSTIMESTAMP,
  pool_name VARCHAR2(30),
  request_size NUMBER,
  error_message VARCHAR2(500),
  sql_text CLOB,
  module VARCHAR2(64),
  action VARCHAR2(64)
);

-- Procedure to monitor shared pool
CREATE OR REPLACE PROCEDURE monitor_shared_pool AS
  v_free_memory NUMBER;
  v_pool_size NUMBER;
  v_free_pct NUMBER;
BEGIN
  -- Get free memory percentage
  SELECT SUM(DECODE(name,'free memory',bytes,0)),
         SUM(bytes)
  INTO v_free_memory, v_pool_size
  FROM v$sgastat
  WHERE pool = 'shared pool';

  v_free_pct := (v_free_memory / v_pool_size) * 100;

  -- Alert if free memory < 5%
  IF v_free_pct < 5 THEN
    DBMS_OUTPUT.PUT_LINE('WARNING: Shared pool free memory at ' ||
                        ROUND(v_free_pct,2) || '%');
    -- Add alerting logic here
  END IF;

  -- Check for recent failures
  FOR rec IN (
    SELECT request_failures, last_failure_size
    FROM v$shared_pool_reserved
    WHERE request_failures > 0
      AND last_failure_size > 0
  ) LOOP
    DBMS_OUTPUT.PUT_LINE('Shared pool failures detected: ' ||
                        rec.request_failures);
  END LOOP;
END;
/

Automated Response

-- Create automated flush job for emergencies
CREATE OR REPLACE PROCEDURE auto_flush_shared_pool AS
  v_free_mb NUMBER;
BEGIN
  -- Get free memory in MB
  SELECT SUM(bytes)/1024/1024 INTO v_free_mb
  FROM v$sgastat
  WHERE pool = 'shared pool' AND name = 'free memory';

  -- If free memory < 50MB, flush
  IF v_free_mb < 50 THEN
    EXECUTE IMMEDIATE 'ALTER SYSTEM FLUSH SHARED_POOL';

    -- Log the action
    INSERT INTO dba_maintenance_log (
      action_date, action_type, action_detail
    ) VALUES (
      SYSTIMESTAMP, 'EMERGENCY_FLUSH',
      'Shared pool flushed due to low memory: ' || v_free_mb || 'MB free'
    );
    COMMIT;
  END IF;
END;
/

Best Practices

Application Development

1. Always use bind variables
2. Limit dynamic SQL generation
3. Close cursors explicitly
4. Use connection pooling
5. Prepare statements once, execute many

Database Configuration

-- Recommended initial settings
ALTER SYSTEM SET shared_pool_size = 2G SCOPE=SPFILE;
ALTER SYSTEM SET shared_pool_reserved_size = 200M SCOPE=SPFILE;
ALTER SYSTEM SET cursor_sharing = EXACT SCOPE=SPFILE;
ALTER SYSTEM SET open_cursors = 300 SCOPE=SPFILE;

-- For OLTP systems with bind variables
ALTER SYSTEM SET session_cached_cursors = 100 SCOPE=SPFILE;

-- Enable memory advisors
ALTER SYSTEM SET statistics_level = TYPICAL SCOPE=SPFILE;

Regular Maintenance

-- Weekly shared pool analysis
SET SERVEROUTPUT ON
DECLARE
  v_sql_reuse_pct NUMBER;
  v_single_use_sql NUMBER;
BEGIN
  -- Check SQL reuse
  SELECT (1 - (single_use_sql/total_sql)) * 100,
         single_use_sql
  INTO v_sql_reuse_pct, v_single_use_sql
  FROM (
    SELECT COUNT(*) total_sql,
           SUM(CASE WHEN executions = 1 THEN 1 ELSE 0 END) single_use_sql
    FROM v$sqlarea
  );

  DBMS_OUTPUT.PUT_LINE('SQL Reuse: ' || ROUND(v_sql_reuse_pct,2) || '%');
  DBMS_OUTPUT.PUT_LINE('Single-use SQL: ' || v_single_use_sql);

  IF v_sql_reuse_pct < 80 THEN
    DBMS_OUTPUT.PUT_LINE('WARNING: Poor SQL reuse indicates bind variable issues');
  END IF;
END;
/

Related Errors

• ORA-04030 - Out of process memory (PGA)
• ORA-00604 - Error occurred at recursive SQL level
• ORA-01000 - Maximum open cursors exceeded
• ORA-12520 - TNS:listener could not find handler

Emergency Response Plan

Quick Actions

1. Flush shared pool (immediate relief)

   ALTER SYSTEM FLUSH SHARED_POOL;

2. Increase memory (if possible)

   ALTER SYSTEM SET shared_pool_size = 3G SCOPE=MEMORY;

3. Kill problematic sessions

   -- Find sessions with many open cursors
   SELECT sid, serial#, username, program
   FROM v$session
   WHERE sid IN (
     SELECT sid FROM v$open_cursor
     GROUP BY sid HAVING COUNT(*) > 200
   );

4. Enable cursor sharing (temporary)

   ALTER SYSTEM SET cursor_sharing = FORCE SCOPE=MEMORY;