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clickhouse-managed-postgres-rca

Testing & Quality
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MUST USE when investigating performance issues on a ClickHouse-managed Postgres instance. Provides an evidence-based RCA workflow that scrapes the Prometheus endpoint for system signal, pulls per-digest evidence from the Slow Query Patterns API, and recommends (does not apply) a fix.

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ClickHouse Managed Postgres RCA

When to use

Trigger whenever a user reports slowness, high CPU, low throughput, cache thrash, or any unexplained pain on a ClickHouse-managed Postgres instance.

What you have access to

Two APIs on https://api.clickhouse.cloud (HTTP Basic auth using a ClickHouse Cloud API key/secret pair):

  • Prometheus metrics — operation postgresInstancePrometheusGet under the Prometheus tag. Returns Prometheus exposition format. System and workload metrics for one Postgres service.
  • Slow Query Patterns — operation slowQueryPatternsGetList under the Postgres tag. Returns per-digest latency, IO, and call statistics for normalized query patterns. Beta.

Both endpoints require an organizationId and a serviceId as path parameters. The user must supply both, plus the API key/secret pair.

What you do NOT have

  • Query plans / EXPLAIN output.
  • Per-table scan-type counters (seq_scan / idx_scan).
  • Autovacuum or last-ANALYZE timestamps.

Reason from IO and timing signals, not from a plan tree.

Workflow

Six steps, in order. Do not skip ahead.

Steps 2 and 3 only share auth — no data dependency between them. Run them in parallel (background curls, & + wait) to cut wall time from sequential ~2s to ~1s.

1. Discover the live API shape

These endpoints are Beta — paths, params, and JSON field names can shift. Follow rules/openapi-discovery.md to:

  1. Fetch the OpenAPI spec from https://api.clickhouse.cloud/v1.
  2. Locate the two operations by operationId:
    • postgresInstancePrometheusGet (Prometheus tag)
    • slowQueryPatternsGetList (Postgres tag)
  3. Resolve their path templates, required query parameters, and (for the slow-query endpoint) the response schema.
  4. Build a session-scoped role map from the schema property descriptions: { semantic role → actual field name }.

Use the resolved names in every subsequent request and citation. Never hardcode field names from memory.

2. Scrape Prom once for system gauges

Follow rules/prometheus-scrape.md. One scrape, no wait. You're after gauges (current values) that don't need a delta: CacheHitRatio, ActiveConnections, MemoryUsedPercent, FilesystemUsedPercent.

A CacheHitRatio well below ~95% on a workload that should fit in cache is a real signal on its own. Climbing ActiveConnections toward the pool ceiling is a real signal on its own. These don't need rate-of-change.

A second scrape for counter deltas is opt-in, used only when Step 4 triage points at write-congestion (where deadlock and rollback rates matter and the Slow Query Patterns API can't substitute). For the read-path case (the most common RCA shape) the single scrape is enough.

3. Pull top slow query patterns

Request the slow query patterns. Follow rules/slow-query-patterns-fields.md for the fields that matter and how to read them. This is the primary diagnostic — it returns per-pattern accumulated totals (call count, runtime, blocks, rows) over the window you request, which is the "rate-of-change" data you'd otherwise derive from two Prom scrapes — but per query and without waiting.

If no patterns return a meaningful totalDurationUs, the report may be overstated or the issue isn't query-shaped. Stop and tell the user what you looked at.

4. Triage: pick the right heuristic

Follow rules/triage.md. Match the combined Prom + slow-query signal to one of the heuristic shapes. Each shape points to a specific heuristic file:

  • rules/heuristic-full-scan.md — read-path full scan.
  • rules/heuristic-hot-loop.md — N+1 / hot loop from the app.
  • rules/heuristic-write-congestion.md — deadlocks, slow writes, high rollback rate.

If the signal does not match any shape cleanly, do not invent a hypothesis. Surface the top patterns and ask the user which workload they recognize. New heuristics are welcome as PRs.

5. Reason, then recommend

Use the format in rules/output-template.md. Always include: symptom, evidence, hypothesis (noting any alternative cause you cannot rule out from this surface alone), short-term fix, and long-term follow-ups.

6. Do not apply the fix

Follow rules/recommend-only.md. Never run DDL. Never call pg_cancel_backend or pg_terminate_backend. Write the recommendation, explain why, and let the human apply it.

Full Compiled Document

For the complete guide with every rule expanded in a single context load: AGENTS.md.