Feat/1225 add variant metric indexes

[Peolite001]

Summary

• Adds a composite index to speed common lookups on variant metrics and a reviewable migration to create the same index on existing DBs.

Background

• Queries frequently fetch metrics for a variant and iterate them ordered by createdAt (see batch-loading in statistical-analysis.service.ts). The table previously had no index on the variant foreign key + createdAt, which forces sequential scans as the table grows.

Changes

• Entity: variant-metric.entity.ts
  • Added composite index: IDX_variant_metrics_variant_createdAt on (variant, createdAt).
• Migration: -add-variant-metric-indexes.ts
  • Creates/drops index IDX_variant_metrics_variant_createdAt on variant_metrics(variantId, createdAt).

Why this index

• Covers the common query pattern: WHERE variant.id = $id ORDER BY createdAt ASC (and similar range/ordered scans).
• A composite index on (variantId, createdAt) supports both the filter and the sort without needing an additional index.
• Avoids redundant indexes — this is a single composite index rather than separate single-column duplicates.

Migration SQL (what will be created)

• CREATE INDEX IDX_variant_metrics_variant_createdAt ON variant_metrics (variantId, createdAt);
• Rollback: DROP INDEX IDX_variant_metrics_variant_createdAt;

Impact & testing

• Low-risk change: only adds an index and migration. No behavior changes to application logic.
• Performance: read performance for variant-scoped metric queries should improve; write overhead is minimal (single additional index).
• Tests: existing unit tests should be unaffected; run npm run test if you want full validation after applying migrations.

Notes

• Migration file: -add-variant-metric-indexes.ts

Closes #1225

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[RUKAYAT-CODER]

Well done on the job done so far!
Kindly fix workflow to pass.

[Peolite001]

Well done on the job done so far! Kindly fix workflow to pass.

done

[RUKAYAT-CODER]

There are still some lint errors. Kindly fix