perf: Add composite index on jobs table for efficient job fetching

[dimitri-yatsenko]

Summary

Add a composite index to the jobs table to optimize the critical job fetching query used during populate(reserve_jobs=True).

Problem

When populating with job reservation, the following query runs frequently:

SELECT ... FROM jobs
WHERE status='pending' AND scheduled_time <= NOW()
ORDER BY priority ASC, scheduled_time ASC
LIMIT N

Without an index, this requires a full table scan and filesort, which becomes slow with large job queues (thousands of jobs).

Solution

Add a composite index that covers both the WHERE clause and ORDER BY:

INDEX (status, priority, scheduled_time)

This enables:

• Index range scan on status='pending'
• Filtering on scheduled_time within the index
• Index-ordered retrieval by priority, scheduled_time (no filesort needed)

Testing

• All test_jobs.py tests pass
• Verified index creation in MySQL with SHOW INDEX

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