AWS Database Cost Management: Reserved Instances, Idle DB Shutdown, and Storage

# List all RDS instances with CPU metrics
aws cloudwatch get-metric-statistics \
  --namespace AWS/RDS \
  --metric-name CPUUtilization \
  --dimensions Name=DBInstanceIdentifier,Value=my-db \
  --start-time 2025-10-01T00:00:00Z \
  --end-time 2025-10-15T00:00:00Z \
  --period 86400 \
  --statistics Average,Maximum

# Use AWS Compute Optimizer for RDS recommendations
aws compute-optimizer get-rds-database-recommendations \
  --region us-east-1 \
  --filters Name=finding,Values=Overprovisioned

# Lambda + EventBridge: stop RDS at 7pm, start at 8am weekdays
aws events put-rule \
  --name stop-dev-db \
  --schedule-expression 'cron(0 19 ? * MON-FRI *)' \
  --state ENABLED

# Lambda function
import boto3
def handler(event, context):
    rds = boto3.client('rds')
    rds.stop_db_instance(DBInstanceIdentifier='dev-postgres')

Instance: db.r6g.large in us-east-1
On-Demand:   $0.24/hour  = $2,102/year
1-year RI:   $0.151/hour = $1,323/year  (37% savings)
3-year RI:   $0.103/hour = $902/year    (57% savings)

-- PostgreSQL: find tables that could be shrunk
SELECT tablename,
       pg_size_pretty(pg_total_relation_size(schemaname||'.'||tablename)) AS size,
       n_dead_tup,
       last_vacuum
FROM pg_stat_user_tables
WHERE n_dead_tup > 100000
ORDER BY n_dead_tup DESC;

-- After VACUUM FULL or pg_repack, storage cannot be reclaimed in RDS
-- without a snapshot restore to a smaller storage allocation.
-- Consider using pg_repack to reclaim pages for reuse instead.