perf(scopes): stop cross-joining packets in scope stats

GetScopeStats left-joined packets, observer_scopes and nodes at once,
multiplying rows into the millions before COUNT(DISTINCT) deduped them
(~10s per call). Count each table on its own and index packets(scope_id).

db/migrations/013_packets_scope_index.sql

@@ -0,0 +1,3 @@
+-- Scope stats count off scope_id, which had no index.
+
+CREATE INDEX idx_packets_scope ON packets(scope_id) WHERE scope_id IS NOT NULL;

db/queries/queries.sql

@@ -917,16 +917,14 @@ WHERE ($1::text = '' OR preset = $1::text)
 ORDER BY preset, iata, source_type;
 
 -- name: GetScopeStats :many
+-- Count each table on its own; the old cross-join blew up to millions of rows
+-- before COUNT(DISTINCT) (~10s).
 SELECT
     ts.name,
-    COUNT(DISTINCT p.packet_hash) AS packet_count,
-    COUNT(DISTINCT os.observer_id) AS observer_count,
-    COUNT(DISTINCT n.id) AS node_count
+    (SELECT COUNT(*) FROM packets p WHERE p.scope_id = ts.id) AS packet_count,
+    (SELECT COUNT(*) FROM observer_scopes os WHERE os.scope_id = ts.id) AS observer_count,
+    (SELECT COUNT(*) FROM nodes n WHERE n.default_scope_id = ts.id) AS node_count
 FROM transport_scopes ts
-LEFT JOIN packets p ON p.scope_id = ts.id
-LEFT JOIN observer_scopes os ON os.scope_id = ts.id
-LEFT JOIN nodes n ON n.default_scope_id = ts.id
-GROUP BY ts.name
 ORDER BY ts.name;
 
 -- ============================================================

db/sqlc/querier.go

@@ -50,6 +50,8 @@ type Querier interface {
 	GetRegionIATAs(ctx context.Context, regionID int32) ([]string, error)
 	GetScopeByName(ctx context.Context, name string) (GetScopeByNameRow, error)
 	GetScopeNames(ctx context.Context) ([]string, error)
+	// Count each table on its own; the old cross-join blew up to millions of rows
+	// before COUNT(DISTINCT) (~10s).
 	GetScopeStats(ctx context.Context) ([]GetScopeStatsRow, error)
 	GetScopesByIATAs(ctx context.Context, dollar_1 []string) ([]GetScopesByIATAsRow, error)
 	// Returns node counts grouped by type, optionally filtered by IATA.

db/sqlc/queries.sql.go

@@ -1035,14 +1035,10 @@ func (q *Queries) GetScopeNames(ctx context.Context) ([]string, error) {
 const getScopeStats = `-- name: GetScopeStats :many
 SELECT
     ts.name,
-    COUNT(DISTINCT p.packet_hash) AS packet_count,
-    COUNT(DISTINCT os.observer_id) AS observer_count,
-    COUNT(DISTINCT n.id) AS node_count
+    (SELECT COUNT(*) FROM packets p WHERE p.scope_id = ts.id) AS packet_count,
+    (SELECT COUNT(*) FROM observer_scopes os WHERE os.scope_id = ts.id) AS observer_count,
+    (SELECT COUNT(*) FROM nodes n WHERE n.default_scope_id = ts.id) AS node_count
 FROM transport_scopes ts
-LEFT JOIN packets p ON p.scope_id = ts.id
-LEFT JOIN observer_scopes os ON os.scope_id = ts.id
-LEFT JOIN nodes n ON n.default_scope_id = ts.id
-GROUP BY ts.name
 ORDER BY ts.name
 `
 
@@ -1053,6 +1049,8 @@ type GetScopeStatsRow struct {
 	NodeCount     int64  `json:"node_count"`
 }
 
+// Count each table on its own; the old cross-join blew up to millions of rows
+// before COUNT(DISTINCT) (~10s).
 func (q *Queries) GetScopeStats(ctx context.Context) ([]GetScopeStatsRow, error) {
 	rows, err := q.db.Query(ctx, getScopeStats)
 	if err != nil {