doctor: complete runtime check documentation sprint

Signed-off-by: master <>

docs/doctor/articles/postgres/db-latency.md

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+---
+checkId: check.db.latency
+plugin: stellaops.doctor.database
+severity: fail
+tags: [database, postgres, latency, performance]
+---
+# Query Latency
+
+## What It Checks
+Runs two warmup queries and then measures five `SELECT 1` probes plus five temporary-table `INSERT` probes against PostgreSQL.
+
+The check warns when the p95 latency exceeds `50ms` and fails when the p95 latency exceeds `200ms`.
+
+## Why It Matters
+Healthy connectivity is not enough if the database path is slow. Elevated query latency turns into slow UI pages, delayed releases, and queue backlogs across the platform.
+
+## Common Causes
+- CPU, memory, or I/O pressure on the PostgreSQL host
+- Cross-host or cross-region latency between Doctor and PostgreSQL
+- Lock contention or long-running transactions
+- Shared infrastructure saturation in the default compose stack
+
+## How to Fix
+
+### Docker Compose
+```bash
+docker compose -f devops/compose/docker-compose.stella-ops.yml exec postgres psql -U stellaops -d stellaops -c "SELECT * FROM pg_stat_activity WHERE state = 'active';"
+docker compose -f devops/compose/docker-compose.stella-ops.yml exec postgres psql -U stellaops -d stellaops -c "SELECT * FROM pg_locks WHERE NOT granted;"
+docker compose -f devops/compose/docker-compose.stella-ops.yml stats postgres
+```
+
+Tune connection placement and storage before raising thresholds. If the database is remote, keep `doctor-web` and PostgreSQL on the same low-latency network segment.
+
+### Bare Metal / systemd
+```bash
+psql -h <db-host> -U <db-user> -d <db-name> -c "SELECT * FROM pg_stat_activity WHERE state = 'active';"
+psql -h <db-host> -U <db-user> -d <db-name> -c "SELECT * FROM pg_locks WHERE NOT granted;"
+```
+
+### Kubernetes / Helm
+```bash
+kubectl top pod -n <namespace> <postgres-pod>
+kubectl exec -n <namespace> <postgres-pod> -- psql -U <db-user> -d <db-name> -c "SELECT now();"
+```
+
+## Verification
+```bash
+stella doctor --check check.db.latency
+```
+
+## Related Checks
+- `check.db.connection` - basic reachability must pass before latency numbers are meaningful
+- `check.db.pool.health` - pool saturation often shows up as latency first