feat(db): enhance PostgreSQL statistics monitoring and error handling

Updated the PostgreSQL monitoring service to improve handling of `pg_stat_statements` availability. Introduced a new method to check if the extension is queryable and updated the response structure to include availability status and hints. Enhanced the documentation to clarify the requirements for enabling `pg_stat_statements`. Adjusted related components to reflect these changes, ensuring better user feedback in the monitoring interface.

docs/db-diagnostics.md

@@ -21,7 +21,14 @@ psql "$EVOBGP_DATABASE_URL"
 
 CLI на CP: `evobgp-api db report|vacuum|analyze|cleanup` (см. `internal/dbcli`).
 
-Миграция `000023` создаёт `pg_stat_statements`; в production может потребоваться `shared_preload_libraries` и перезапуск Postgres.
+Миграция `000023` создаёт `pg_stat_statements`; для сбора статистики **обязательно** preload и перезапуск Postgres:
+
+```text
+# postgresql.conf или command в compose
+shared_preload_libraries = 'pg_stat_statements'
+```
+
+После изменения — restart контейнера/сервиса Postgres. Без этого API `/v1/monitoring/postgres/queries` вернёт пустой список (`statements_available: false`), без 5xx.
 
 ## 1. Размеры таблиц и индексов
 

internal/pgmonitor/queries.go

@@ -106,11 +106,7 @@ func (s *Service) fetchOverview(ctx context.Context) (Overview, error) {
 		}
 	}
 
-	var ext bool
-	_ = s.pool.QueryRow(ctx, `
-		SELECT EXISTS (
-		  SELECT 1 FROM pg_extension WHERE extname = 'pg_stat_statements')`).Scan(&ext)
-	out.StatementsEnabled = ext
+	out.StatementsEnabled = s.statementsQueryable(ctx)
 
 	return out, nil
 }
@@ -185,27 +181,75 @@ func (s *Service) TopQueries(ctx context.Context, limit int) (QueriesResponse, e
 	if snap, ok, err := s.loadSnapshot(ctx, "slow_queries", 15*time.Minute); err == nil && ok {
 		var items []QueryStat
 		if err := decodePayload(snap.Payload, &items); err == nil {
-			return QueriesResponse{CollectedAt: snap.CollectedAt, Source: "snapshot", Items: items}, nil
+			return QueriesResponse{
+				CollectedAt:         snap.CollectedAt,
+				Source:              "snapshot",
+				Items:               items,
+				StatementsAvailable: true,
+			}, nil
 		}
 	}
 
-	if !s.statementsEnabled(ctx) {
-		return QueriesResponse{CollectedAt: now, Source: "live", Items: nil}, nil
+	if !s.statementsQueryable(ctx) {
+		return queriesUnavailable(now), nil
 	}
 	items, err := queryTopStatements(ctx, s.pool, limit)
 	if err != nil {
+		if isPgStatStatementsUnavailable(err) {
+			s.markStatementsUnavailable()
+			return queriesUnavailable(now), nil
+		}
 		return QueriesResponse{}, err
 	}
-	return QueriesResponse{CollectedAt: now, Source: "live", Items: items}, nil
+	return QueriesResponse{
+		CollectedAt:         now,
+		Source:              "live",
+		Items:               items,
+		StatementsAvailable: true,
+	}, nil
 }
 
-func (s *Service) statementsEnabled(ctx context.Context) bool {
-	var ok bool
-	_ = s.pool.QueryRow(ctx, `
-		SELECT EXISTS (SELECT 1 FROM pg_extension WHERE extname = 'pg_stat_statements')`).Scan(&ok)
+func queriesUnavailable(at time.Time) QueriesResponse {
+	return QueriesResponse{
+		CollectedAt:         at,
+		Source:              "unavailable",
+		Items:               nil,
+		StatementsAvailable: false,
+		StatementsHint:      statementsUnavailableHint,
+	}
+}
+
+const statementsUnavailableHint = "pg_stat_statements requires shared_preload_libraries and PostgreSQL restart (see docs/db-diagnostics.md)"
+
+// statementsQueryable returns true only when pg_stat_statements can be queried (not merely installed).
+func (s *Service) statementsQueryable(ctx context.Context) bool {
+	if s == nil || s.pool == nil {
+		return false
+	}
+	if v, ok := s.cache.get("stmt_queryable"); ok {
+		if b, ok := v.(bool); ok {
+			return b
+		}
+	}
+	ok := probePgStatStatements(ctx, s.pool)
+	s.cache.set("stmt_queryable", ok)
 	return ok
 }
 
+func (s *Service) markStatementsUnavailable() {
+	s.cache.set("stmt_queryable", false)
+}
+
+func probePgStatStatements(ctx context.Context, pool *pgxpool.Pool) bool {
+	var dummy int64
+	err := pool.QueryRow(ctx, `
+		SELECT COALESCE(SUM(calls), 0)::bigint FROM pg_stat_statements LIMIT 1`).Scan(&dummy)
+	if err == nil {
+		return true
+	}
+	return !isPgStatStatementsUnavailable(err)
+}
+
 func queryTopStatements(ctx context.Context, pool *pgxpool.Pool, limit int) ([]QueryStat, error) {
 	rows, err := pool.Query(ctx, `
 		SELECT queryid, left(query, 500), calls, total_exec_time, mean_exec_time, rows
@@ -214,7 +258,7 @@ func queryTopStatements(ctx context.Context, pool *pgxpool.Pool, limit int) ([]Q
 		ORDER BY mean_exec_time DESC
 		LIMIT $1`, limit)
 	if err != nil {
-		if isUndefinedTable(err) {
+		if isPgStatStatementsUnavailable(err) {
 			return nil, nil
 		}
 		return nil, fmt.Errorf("pgmonitor: pg_stat_statements: %w", err)
@@ -231,12 +275,24 @@ func queryTopStatements(ctx context.Context, pool *pgxpool.Pool, limit int) ([]Q
 	return out, rows.Err()
 }
 
-func isUndefinedTable(err error) bool {
+// isPgStatStatementsUnavailable reports extension missing or not loaded via shared_preload_libraries.
+func isPgStatStatementsUnavailable(err error) bool {
+	if err == nil {
+		return false
+	}
 	var pgErr *pgconn.PgError
 	if errors.As(err, &pgErr) {
-		return pgErr.Code == "42P01" || pgErr.Code == "42704"
+		switch pgErr.Code {
+		case "42P01", "42704", "55000":
+			return true
+		}
+		msg := strings.ToLower(pgErr.Message)
+		if strings.Contains(msg, "shared_preload_libraries") || strings.Contains(msg, "pg_stat_statements") {
+			return true
+		}
 	}
-	return strings.Contains(err.Error(), "pg_stat_statements")
+	low := strings.ToLower(err.Error())
+	return strings.Contains(low, "shared_preload_libraries") || strings.Contains(low, "pg_stat_statements")
 }
 
 func isSafeIdent(name string) bool {

internal/pgmonitor/scheduler.go

@@ -33,7 +33,7 @@ func StartScheduler(ctx context.Context, pool *pgxpool.Pool) {
 			c, cancel := context.WithTimeout(context.Background(), 3*time.Minute)
 			defer cancel()
 			if err := s.AggregateSlowQueries(c, 30); err != nil {
-				log.Printf("pgmonitor: slow queries: %v", err)
+				log.Printf("pgmonitor: slow queries snapshot: %v", err)
 			}
 			if err := s.EstimateTableBloat(c); err != nil {
 				log.Printf("pgmonitor: bloat: %v", err)

internal/pgmonitor/service_test.go

@@ -1,6 +1,11 @@
 package pgmonitor
 
-import "testing"
+import (
+	"errors"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgconn"
+)
 
 func TestClampLimit(t *testing.T) {
 	if clampLimit(0, 20, 100) != 20 {
@@ -31,3 +36,13 @@ func TestNewServiceNilPool(t *testing.T) {
 		t.Fatal("expected nil service")
 	}
 }
+
+func TestIsPgStatStatementsUnavailable(t *testing.T) {
+	err := &pgconn.PgError{Code: "55000", Message: "pg_stat_statements must be loaded via shared_preload_libraries"}
+	if !isPgStatStatementsUnavailable(err) {
+		t.Fatal("55000")
+	}
+	if isPgStatStatementsUnavailable(errors.New("other")) {
+		t.Fatal("unrelated")
+	}
+}

internal/pgmonitor/snapshot.go

@@ -66,11 +66,15 @@ func (s *Service) RefreshMetricsSnapshot(ctx context.Context) error {
 
 // AggregateSlowQueries stores top statements snapshot.
 func (s *Service) AggregateSlowQueries(ctx context.Context, limit int) error {
-	if !s.statementsEnabled(ctx) {
+	if !s.statementsQueryable(ctx) {
 		return s.UpsertSnapshot(ctx, "slow_queries", []QueryStat{})
 	}
 	items, err := queryTopStatements(ctx, s.pool, clampLimit(limit, 20, 100))
 	if err != nil {
+		if isPgStatStatementsUnavailable(err) {
+			s.markStatementsUnavailable()
+			return s.UpsertSnapshot(ctx, "slow_queries", []QueryStat{})
+		}
 		return err
 	}
 	return s.UpsertSnapshot(ctx, "slow_queries", items)

internal/pgmonitor/types.go

@@ -71,9 +71,11 @@ type QueryStat struct {
 
 // QueriesResponse for GET /monitoring/postgres/queries.
 type QueriesResponse struct {
-	CollectedAt time.Time   `json:"collected_at"`
-	Source      string      `json:"source"` // live | snapshot
-	Items       []QueryStat `json:"items"`
+	CollectedAt          time.Time   `json:"collected_at"`
+	Source               string      `json:"source"` // live | snapshot | unavailable
+	Items                []QueryStat `json:"items"`
+	StatementsAvailable  bool        `json:"statements_available"`
+	StatementsHint       string      `json:"statements_hint,omitempty"`
 }
 
 // LockRow describes a lock / blocking session.

web/src/lib/components/monitoring/MonitoringPostgresTab.svelte

@@ -281,12 +281,18 @@
 					<CardTitle>Медленные запросы</CardTitle>
 					<CardDescription>
 						Источник: {queries?.source ?? '—'}
-						{#if overview && !overview.pg_stat_statements_enabled}
-							· pg_stat_statements не включён
+						{#if queries?.statements_available === false || (overview && !overview.pg_stat_statements_enabled)}
+							· pg_stat_statements недоступен
 						{/if}
 					</CardDescription>
 				</CardHeader>
 				<CardContent>
+					{#if queries?.statements_hint}
+						<Alert class="mb-4">
+							<AlertTitle>Нет статистики запросов</AlertTitle>
+							<AlertDescription>{queries.statements_hint}</AlertDescription>
+						</Alert>
+					{/if}
 					<Table>
 						<TableHeader>
 							<TableRow>

web/src/lib/monitoring/postgres.ts

@@ -48,6 +48,8 @@ export type PostgresQueriesResponse = {
 	collected_at: string;
 	source: string;
 	items: PostgresQueryRow[];
+	statements_available?: boolean;
+	statements_hint?: string;
 };
 
 export type PostgresLockRow = {