Improve test coverage for clickhouse package (best covered but gaps remain)

[BrewingCoder]

Area

Backend — Database / Migrations

Category

Test coverage

Description

The clickhouse package has the best test coverage in the backend (~2,450 lines of tests) but still has significant gaps in the 7,800+ LOC codebase.

Coverage gaps:

1. Query builder SQL generation with complex filters — limited edge case testing
2. Sampling table selection logic (>20M row threshold)
3. Write operations (batch insert correctness for all 5 data streams)
4. Connection failover (read-only connection fallback)
5. Migration idempotency verification

Structural concerns:

• query.go is 2,181 lines — largest file in the backend. Should be split by data stream.
• 146 migrations with no rollback testing
• Timestamp precision mismatch: logs (seconds) vs traces (nanoseconds)

See src/backend/clickhouse/MODULE.md for full analysis.

Affected Files / Packages

src/backend/clickhouse/query.go (2,181 lines — needs splitting)
src/backend/clickhouse/sessions.go (943 lines)
src/backend/clickhouse/errors.go (881 lines)
src/backend/clickhouse/traces.go (678 lines)
src/backend/clickhouse/logs.go (516 lines)
+ 11 more implementation files

[github-actions]

AI Triage: Improve test coverage for ClickHouse package by addressing gaps in query builder edge cases, sampling logic, write operations, failover handling, and migration idempotency.

Labels applied: area:backend, tech-debt, priority:medium, component:database