Database Observability

Slow Query Logging

The backend instruments every PostgreSQL query via a pgx.QueryTracer attached to the connection pool. When a query's wall-clock duration exceeds the configured threshold a structured WARN log entry is emitted.

Configuration

Environment variable	Type	Default	Description
SLOW_QUERY_THRESHOLD_MS	integer (ms)	500	Minimum duration that triggers a slow-query log. Set to 0 to disable log emission while keeping metrics active.

Log format

{
  "time": "2026-04-24T12:00:00Z",
  "level": "WARN",
  "msg": "slow query detected",
  "duration_ms": 823,
  "threshold_ms": 500,
  "sql": "SELECT * FROM contributions WHERE user_id = $1"
}

• sql — the query template only. Bind parameters are never logged to prevent sensitive data (PII, secrets) from appearing in log streams.
• error — present only when the query also returned an error.

Security assumptions

• Query arguments (Args) are captured by pgx in TraceQueryStartData but the tracer discards them immediately; they are never stored or forwarded.
• The SQL template itself may contain table/column names. Avoid embedding literal values directly in SQL strings.
• maskDBURL redacts the password from the connection string before it is logged at startup.

Metrics

In-process counters are exported via Go's standard expvar package and are available at GET /debug/vars (net/http default mux, enabled in dev; gate behind auth in production).

Metric	Type	Description
db_queries_total	counter	Total queries executed (all durations).
db_slow_queries_total	counter	Queries that exceeded the threshold.
db_query_duration_ms_total	counter	Cumulative query duration in milliseconds.

Derived rate (example with any scraper):

slow_query_rate = db_slow_queries_total / db_queries_total
avg_duration_ms = db_query_duration_ms_total / db_queries_total

Disabling slow-query logging (metrics only)

SLOW_QUERY_THRESHOLD_MS=0

All three counters still increment; only the WARN log is suppressed.

Implementation notes

• Zero extra dependencies — uses expvar (stdlib) and log/slog (stdlib).
• pgx tracer interface — SlowQueryTracer implements pgx.QueryTracer (TraceQueryStart / TraceQueryEnd). It is attached via pgxpool.Config.ConnConfig.Tracer.
• Atomic disabled flag — the hot path checks an atomic.Bool to avoid allocations when logging is disabled.
• Context propagation — start time is stored in the request context between the two tracer calls; no global state per query.