Optimize database queries: eliminate N+1 problems and batch operations

[Copilot]

Identified and fixed multiple performance bottlenecks causing excessive database round-trips in message publishing, queue operations, and subscription lookups.

Changes

Batch queue fetching in topic message publishing

• Added QueueRepository.GetMany(ids []string) to fetch multiple queues in one query
• Modified Topic.CreateMessage() to collect queue IDs and fetch in batch
• Impact: Reduces N+1 queries to 2 queries when publishing to N subscriptions

// Before: N queries in loop
for _, subscription := range subscriptions {
    queue, err := t.queueRepository.Get(ctx, subscription.QueueID)
    // ...
}

// After: 1 batch query
queues, err := t.queueRepository.GetMany(ctx, queueIDs)

Batch message updates on list operations

• Replaced per-message UPDATEs with single batch UPDATE using ANY() operator
• Maintains in-memory state updates for returned message objects
• Impact: Reduces N UPDATE queries to 1 when listing N messages

-- Single query replaces N individual updates
UPDATE messages 
SET delivery_attempts = delivery_attempts + 1, 
    scheduled_at = $1, 
    updated_at = $2 
WHERE id = ANY($3)

Database index for subscription lookups

• Added B-tree index on subscriptions.topic_id
• Migration: 000003_add_subscriptions_topic_idx

Removed redundant Get calls in service layer

• Eliminated validation queries in Delete, Stats, Purge, Cleanup operations
• Database operations already fail appropriately when entity doesn't exist
• Impact: Reduces 2 queries to 1 per operation

Performance Characteristics

Operation	Before	After	Reduction
Publish to 100 subscriptions	101 queries	2 queries	98%
List 50 messages	51 queries	2 queries	96%
Queue delete/stats/purge/cleanup	2 queries	1 query	50%

Original prompt

Identify and suggest improvements to slow or inefficient code

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