perf(database): index ProjectAlert.channelId so alert-channel deletes stop seq-scanning

[ericallam]

Why this change

Deleting a ProjectAlertChannel fires the FK cascade DELETE FROM ONLY "ProjectAlert" WHERE $1 = "channelId". That cascade is scan-shaped: with no index on channelId, it reads the entire ProjectAlert table to find the few child rows belonging to the deleted channel. The parent DELETE ProjectAlertChannel does almost no work itself; its latency is dominated by this cascade. ProjectAlert is append-heavy and grows over time, so the scan cost only increases.

Diagnosis

ProjectAlert had no index on channelId (only pkey + a friendlyId unique). The cascade therefore did a full sequential scan of the whole table. The sibling ProjectAlertStorage cascade on the same delete is index-backed and stays fast, which isolates the missing index as the cause.

Change

Add @@index([channelId]) on ProjectAlert, created with CREATE INDEX CONCURRENTLY IF NOT EXISTS so prisma migrate deploy stays safe on a live table.

Benchmark (local, seeded)

Local Postgres seeded with 1,000,000 ProjectAlert rows across 50 channels (~20k rows per channel), EXPLAIN (ANALYZE, BUFFERS) on the cascade delete:

	before	after
plan	Seq Scan (1M rows)	Bitmap Index Scan
direct child delete	740 ms	22 ms
parent delete ProjectAlert_channelId_fkey trigger	77.7 ms	23.8 ms

Expected impact

The cascade drops from a full-table sequential scan to a targeted index lookup. The win grows with the table: the more rows in ProjectAlert, the more a scan costs and the more the index saves, so the benefit is larger than the seeded numbers above.

Risks

• One extra btree to maintain on every ProjectAlert insert; acceptable for a single-column index on a high-insert table, and it should be pre-created before the migration deploys (per the repo index rules).
• No behavior change: no rows orphaned, no ordering or result-set change, read paths untouched.

Follow-up

ProjectAlert's other cascade FK columns (projectId, environmentId, workerDeploymentId) are also unindexed, but their parents are soft-deleted rather than physically removed, so those cascades do not currently fire. Lower priority unless a hard-delete path is introduced.

[changeset-bot]

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Latest commit: a059460

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[coderabbitai]

Walkthrough

Adds an index on ProjectAlert.channelId to the Prisma schema. Adds a concurrent, idempotent migration to create the database index. Documents faster alert-channel deletion as alert history grows.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains the problem, diagnosis, change, benchmark, impact, risks, and follow-up, but it omits the required issue, checklist, changelog, and screenshots sections.	Add the template sections, including the issue reference, completed checklist, explicit testing steps, changelog entry, and screenshots or a statement that screenshots are not applicable.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the primary database performance change and its effect on ProjectAlertChannel deletes.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/tri-13095-index-projectalertchannelid-so-alert-channel-deletes-stop

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.