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Oracle Autonomous Database - Expert Knowledge

NEVER Do This

NEVER use ADMIN user in application code

ADMIN has full database control; audit trail shows all actions as ADMIN (no accountability); ADMIN cannot be locked/disabled without breaking automation.

-- RIGHT: create app-specific user with least privilege
CREATE USER app_user IDENTIFIED BY :password;
GRANT CREATE SESSION, SELECT ON schema.table TO app_user;

NEVER scale ECPUs without checking wait events first

Scaling 2→4 ECPU costs $526/month extra. If root cause is bad SQL, that is wasted money.

Decision path:
1. Check v$system_event for top wait events
2. High 'CPU time' → Bad SQL, optimize first (do NOT scale)
3. High 'db file sequential read' → Missing indexes (do NOT scale)
4. High 'User I/O' sustained → Scale storage IOPS OR enable auto-scaling
5. Only scale ECPUs if: CPU wait sustained + SQL already optimized

NEVER assume stopped ADB = zero cost

Stopped ADB charges:
  Compute: $0 (stopped)
  Storage: $0.025/GB/month CONTINUES
  Backups: Retention charges CONTINUE

For long-term idle (>60 days): Export via Data Pump, delete ADB, restore from backup.

NEVER create manual backups without retention (kept forever)

# WRONG - charged $0.025/GB/month FOREVER
oci db autonomous-database-backup create \
  --autonomous-database-id $ADB_ID \
  --display-name "pre-upgrade-backup"

# RIGHT - set retention
oci db autonomous-database-backup create \
  --autonomous-database-id $ADB_ID \
  --display-name "pre-upgrade-backup" \
  --retention-days 30
# 1TB × $0.025 × 12 months = $300/year if forgotten

NEVER enable auto-scaling without setting a max ECPU limit

Auto-scaling bills for PEAK usage each hour.
Base 2 ECPU → can scale to 6 ECPU (3× hard limit).
Without max cap: $526/month → $1,578/month surprise.

RIGHT: Set Max ECPU = 4 in console (2× base) to cap at $1,052/month.

NEVER use ROWNUM with ORDER BY (wrong results)

-- WRONG: ROWNUM applied BEFORE ORDER BY
SELECT * FROM orders WHERE ROWNUM <= 10 ORDER BY created_at DESC;

-- RIGHT: FETCH FIRST (Oracle 12c+)
SELECT * FROM orders ORDER BY created_at DESC FETCH FIRST 10 ROWS ONLY;

Performance Troubleshooting Decision Tree

"Queries are slow"
│
├─ ONE query slow?
│  └─ Get SQL_ID → check execution plan:
│     SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR('&sql_id'));
│     ├─ TABLE ACCESS FULL on large table → Add index
│     ├─ Wrong join order → SQL hints or SQL Plan Baseline
│     └─ Cartesian join → Fix query logic
│
├─ ALL queries slow (system-wide)?
│  └─ Check wait events:
│     SELECT event, time_waited_micro/1000000 AS wait_sec
│     FROM v$system_event WHERE wait_class != 'Idle'
│     ORDER BY time_waited_micro DESC FETCH FIRST 10 ROWS ONLY;
│     ├─ 'CPU time' → Optimize SQL OR scale ECPU (check SQL first)
│     ├─ 'db file sequential read' → Missing indexes
│     ├─ 'db file scattered read' → Full table scans
│     ├─ 'log file sync' → Too many commits (batch DML)
│     └─ 'User I/O' → Scale storage IOPS or enable auto-scaling
│
└─ When did it start?
   ├─ After schema change → DBMS_STATS.GATHER_TABLE_STATS
   ├─ After data load → Gather stats + check partitioning
   ├─ After version upgrade → Compare execution plans
   └─ Gradual over time → Data growth, need indexing/partitioning

SQL_ID Debugging Workflow

Step 1: Find problem SQL_ID

SELECT sql_id, elapsed_time/executions/1000 AS avg_ms,
       executions, sql_text
FROM v$sql
WHERE executions > 0
  AND last_active_time > SYSDATE - 1/24  -- last hour
ORDER BY elapsed_time DESC
FETCH FIRST 10 ROWS ONLY;

Step 2: Get execution plan

SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR('&sql_id'));

Step 3: Create and run SQL Tuning Task

DECLARE task_name VARCHAR2(30);
BEGIN
  task_name := DBMS_SQLTUNE.CREATE_TUNING_TASK(
    sql_id => '&sql_id', task_name => 'tune_slow_query');
  DBMS_SQLTUNE.EXECUTE_TUNING_TASK(task_name);
END;
/
SELECT DBMS_SQLTUNE.REPORT_TUNING_TASK('tune_slow_query') FROM DUAL;

Step 4: Implement fix

• Recommendation: Add index → create index
• Recommendation: Use hint → test, then fix via SQL Plan Baseline
• Recommendation: Gather stats → EXEC DBMS_STATS.GATHER_TABLE_STATS('schema','table')

ADB-Specific Behaviors

Auto-scaling hard limits (cannot change):

Minimum: 1× base ECPU
Maximum: 3× base ECPU
Scale-up trigger: CPU > 80% for 5+ minutes
Scale-down trigger: CPU < 60% for 10+ minutes
Time to scale: 5-10 minutes
Billing: charged for PEAK usage each hour

ADMIN user restrictions in ADB (differs from on-premises):

CANNOT: Create tablespaces (DATA auto-managed)
CANNOT: Modify SYSTEM/SYSAUX tablespaces
CANNOT: Access OS (no shell, no file system)
CANNOT: Use SYSDBA privileges (not available in ADB)

Service name performance impact:

Gotcha: Using HIGH for background jobs starves interactive users with no extra cost benefit.

Backup retention (automatic vs manual):

Automatic: Daily incremental + weekly full, 60-day default, INCLUDED in storage cost
Manual: On-demand, FOREVER retention until manually deleted, $0.025/GB/month
Cost trap: 10 manual backups × 1TB × $0.025 = $250/month ongoing

Version Feature Matrix

Upgrade path: 19c → 21c → 23ai → 26ai (downgrade NOT supported) Rule: Always test in clone before upgrading production.

Common ADB Errors

Reference Files

Load references/oci-cli-adb.md when:

• Provisioning, scaling, or deleting ADB instances
• Creating backups or clones (full vs metadata)
• Downloading wallet files
• Changing auto-scaling, license type, or version

Load references/sqlcl-workflows.md when:

• Executing SQL queries via Bash (SQLcl)
• Running DBMS_SQLTUNE tasks
• Data Pump export/import
• Generating DDL for schema objects

Load references/oci-adb-best-practices.md when:

• Designing ADB architecture from scratch
• Planning ATP vs ADW vs APEX vs JSON workload type
• Migrating from on-premises Oracle to ADB

See references/adb-ha-dr.md for: Autonomous Data Guard setup, cross-region DR, RTO/RPO targets.

See references/adb-security.md for: mTLS wallet configuration, private endpoints, VCN Service Gateway setup.

Pricing reference: See references/cost-reference.md for ECPU/storage pricing tables and auto-scaling cost calculations.