Oracle 26ai In-Memory Performance Issue: Why Tables Are Not Populating and How to Fix It

Have you enabled Oracle In-Memory but noticed that your tables are not being populated?

You run queries expecting faster performance, but nothing changes.

This is a common Oracle 26ai In-Memory performance issue in production systems.

Let’s troubleshoot it step by step using a practical DBA approach.

Problem: Table Not Populating in In-Memory

You execute:

ALTER TABLE sales INMEMORY;

Then check:

SELECT segment_name, populate_status
FROM   v$im_segments
WHERE  segment_name = 'SALES';

You see:

SALES   NO POPULATE

Even after some time, the table is not loaded into memory.

Cause: Why This Happens

Typical reasons include:

• In-Memory is not configured

• In-Memory area is full

• Priority is set to NONE

• IMCO process has not triggered population

• Table is too large for available memory

Step 1: Check In-Memory Configuration

SHOW PARAMETER inmemory_size;

Problem Case

INMEMORY_SIZE = 0

👉 In-Memory is not enabled.

Solution 1: Enable In-Memory

ALTER SYSTEM SET inmemory_size = 8G SCOPE=SPFILE;

Restart the database:

SHUTDOWN IMMEDIATE;
STARTUP;

Step 2: Check Available Memory

SELECT pool, alloc_bytes, used_bytes
FROM   v$inmemory_area;

Problem Case

• used_bytes = alloc_bytes

👉 Memory is fully utilized.

Solution 2: Optimize Memory Usage

Option 1: Remove unnecessary objects

ALTER TABLE old_table NO INMEMORY;

Option 2: Increase memory

ALTER SYSTEM SET inmemory_size = 12G SCOPE=SPFILE;

Step 3: Check Population Priority

SELECT segment_name, inmemory_priority
FROM   v$im_segments
WHERE  segment_name = 'SALES';

Problem Case

SALES   NONE

👉 Table will not populate automatically.

Solution 3: Set Priority

ALTER TABLE sales INMEMORY PRIORITY HIGH;

Step 4: Check Population Status

SELECT segment_name, populate_status
FROM   v$im_segments
WHERE  segment_name = 'SALES';

Status Values

• NOT POPULATED

• POPULATING

• COMPLETED

Step 4.1: Check Detailed Population Metrics

SELECT segment_name,
       bytes,
       inmemory_size,
       bytes_not_populated,
       populate_status
FROM   v$im_segments
WHERE  segment_name = 'SALES';

👉 Helps identify partially populated segments.

Step 5: Force Manual Population

EXEC DBMS_INMEMORY.POPULATE('SALES');

To wait for completion:

EXEC DBMS_INMEMORY_ADMIN.POPULATE_WAIT('SALES');

Step 6: Check Background Processes

SELECT program
FROM   v$process
WHERE  program LIKE '%IMCO%';

👉 If IMCO is not running, population will not occur.

Solution 4: Ensure Background Processes Are Healthy

• Verify database instance health

• Restart if necessary

Step 7: Check Table Size vs Available Memory

SELECT segment_name, bytes
FROM   dba_segments
WHERE  owner = 'SCOTT'
AND    segment_name = 'SALES';

Compare with:

SELECT alloc_bytes
FROM   v$inmemory_area;

👉 If table size exceeds memory, full population is not possible.

Solution 5: Use Column-Level In-Memory

ALTER TABLE sales
INMEMORY (product_id, amount_sold);

👉 Load only required columns.

Step 8: Validate In-Memory Usage in Execution Plan

EXPLAIN PLAN FOR
SELECT SUM(amount_sold)
FROM   sales;

SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY());

Expected

TABLE ACCESS INMEMORY FULL

👉 If not shown, In-Memory is not being used by the optimizer.

Real DBA Example

Problem

• SALES table size: 40GB

• In-Memory size: 16GB

• Population never completed

Fix Applied

ALTER TABLE sales
INMEMORY (product_id, amount_sold, region_id)
MEMCOMPRESS FOR QUERY LOW
PRIORITY HIGH;

Result

• Memory usage reduced to ~8GB

• Population completed successfully

• Query execution time reduced from minutes to seconds

Conclusion

Oracle 26ai In-Memory performance issues are usually caused by configuration gaps, memory limits, or incorrect object settings.

By systematically checking:

• In-Memory configuration

• Memory usage

• Population status

• Execution plans
  

DBAs can quickly identify why objects are not being populated and apply the correct fix.

👉 Enabling In-Memory is not enough verification and monitoring are essential to ensure it is actually being used by the optimizer.