PostgresCollector emits no metrics at all — not even pg_scrape_collector_success=0 — when the per-scrape connection fails, while pg_up stays 1 #1387

Version

v0.20.1 (postgres_exporter_build_info{version="0.20.1", revision="867fbcac31cd18c143e244190ea9168cca069827"}), PostgreSQL 17.10.

What happens

When PostgresCollector cannot open its per-scrape connection, it emits nothing at all — not even pg_scrape_collector_success{collector="…"} 0. The failure exists only as a log line.

https://github.com/prometheus-community/postgres_exporter/blob/v0.20.1/collector/collector.go#L183-L195

func (p PostgresCollector) Collect(ch chan<- prometheus.Metric) {
	// copy the instance so that concurrent scrapes have independent instances
	inst := p.instance.copy()

	// Set up the database connection for the collector.
	err := inst.setup()
	defer inst.Close()
	if err != nil {
		p.logger.Error("Error opening connection to database", "err", err)
		return
	}
	p.collectFromConnection(inst, ch)
}

pg_scrape_collector_success is produced by execute(), which is only reached from collectFromConnection. So the return above skips the one metric that is supposed to report a collector problem.

At the same time the other half of the exporter — the Exporter registered alongside PostgresCollector in cmd/postgres_exporter/main.go — holds a persistent connection per server. It therefore keeps reporting pg_up 1 and pg_exporter_last_scrape_error 0 while every --collector.* metric is missing.

Net effect: the exporter reports "up, no scrape errors" while roughly half of its metric surface has silently disappeared. The only in-band signal is the absence of series.

Why it matters (measured)

On 2026-09-07 a shared development PostgreSQL of ours refused new connections for just under three hours (FATAL: sorry, too many clients already, ~39.7k occurrences). Over that window:

Because collector metrics vanish rather than report a failure, an alert written on them (in our case connection saturation, sum(pg_stat_activity_count) / pg_settings_max_connections > 0.7) evaluates to "no data" and resolves itself in the middle of the incident, exactly when the condition is at its worst. pg_up-based and pg_exporter_last_scrape_error-based alerts do not compensate, because those come from the other registration and stay green.

Connection exhaustion is only the most obvious trigger; anything that makes inst.setup() fail (auth change, pooler saturation, TLS) has the same shape.

Suggestion

Make the failure representable in metrics rather than only in logs. Either of:

1. On inst.setup() error, emit pg_scrape_collector_success{collector=…} 0 for every enabled collector before returning; or
2. Add a dedicated gauge, e.g. pg_scrape_connection_error, set to 1 on that path.

(1) needs no new metric name and makes existing dashboards/alerts on pg_scrape_collector_success correct. Happy to send a PR if the maintainers prefer one of the two.