Index the event log's resubmit count with deleted_at (closes #325)

The resubmit count matches resubmitted_from_id against a page of event
ids, and GORM adds deleted_at IS NULL. With no statistics, SQLite
preferred the deleted_at index once the page held five or more ids, so
every event log page read every live event. deleted_at now joins
resubmitted_from_id in idx_events_resubmitted_from_id, as for the other
event-tier indexes, and the plan test checks the statement with a full
page of ids.

Model: opus-5-5

README.md

@@ -1876,6 +1876,7 @@ tags, so `AutoMigrate` creates them on a fresh database:
 | `deliveries`       | `event_id`, `deleted_at`    | The event log, which loads each event's deliveries, and retention, which counts and deletes the deliveries of expired events |
 | `delivery_results` | `delivery_id`, `deleted_at` | The event log, which loads the attempts of a page's deliveries, and retention, which deletes the attempts of expired events |
 | `events`           | `deleted_at`, `created_at`  | The webhook page's statistics, which count recent events |
+| `events`           | `resubmitted_from_id`, `deleted_at` | The event log, which counts the events resubmitted from each event on a page |
 | `events`           | `created_at`                | Retention, which selects expired events by age |
 
 GORM's soft delete adds `deleted_at IS NULL` to these queries; retention
@@ -1883,9 +1884,9 @@ leaves it out. SQLite keeps no statistics on these tables, and without them it
 rates the `deleted_at` index, which every live row matches, above an index on
 a column matched against several values or compared with a range. So every
 index but the last also covers `deleted_at`. It comes second, so that
-retention can use the index without it, except in `events`, where the
-statistics compare `created_at` with a range (`>=`) and SQLite narrows by a
-range only on the last column it uses.
+retention can use the index without it, except in the statistics' `events`
+index, where they compare `created_at` with a range (`>=`) and SQLite narrows
+by a range only on the last column it uses.
 
 #### Common Fields
 

internal/database/event_tier_indexes_test.go

@@ -93,6 +93,7 @@ func TestEventTierQueriesUseTheirIndexes(t *testing.T) {
 		deliveries []database.Delivery
 		results    []database.DeliveryResult
 		depths     []struct{ Depth int }
+		resubmits  []struct{ Total int }
 		removed    []database.TargetTotals
 	)
 
@@ -116,13 +117,29 @@ func TestEventTierQueriesUseTheirIndexes(t *testing.T) {
 		}).Group("target_id, status").Find(&depths), byStatus)
 
 	// The event log: each event's deliveries, then their attempts
-	// (loadEventsWithDeliveries, loadDeliveryResults).
+	// (loadEventsWithDeliveries, loadDeliveryResults), and how many
+	// events were resubmitted from each of the page's (resubmitCounts).
 	assertPlanUses(t, db, dry.Where("event_id = ?", ids[0]).
 		Find(&deliveries), byEvent)
 	assertPlanUses(t, db, dry.Where("delivery_id IN ?", ids).
 		Order("attempt_num ASC").Find(&results),
 		"idx_delivery_results_delivery_id (delivery_id=? AND deleted_at=?)")
 
+	// The resubmit count gets a full page of 25 ids: with an index on
+	// resubmitted_from_id alone, SQLite still uses it for three ids and
+	// turns to the deleted_at index only from five.
+	page := make([]string, 25)
+	for i := range page {
+		page[i] = uuid.New().String()
+	}
+
+	assertPlanUses(t, db, dry.Model(&database.Event{}).
+		Select("resubmitted_from_id, count(*) AS total").
+		Where("resubmitted_from_id IN ?", page).
+		Group("resubmitted_from_id").Find(&resubmits),
+		"idx_events_resubmitted_from_id "+
+			"(resubmitted_from_id=? AND deleted_at=?)")
+
 	// Retention (reapExpired, deleteEvents): one batch of expired
 	// events, then their attempts, deliveries and the events.
 	var expired []string

internal/database/model_event.go

@@ -19,8 +19,10 @@ type Event struct {
 	// narrows by a < only on the last column it uses. Its final delete
 	// has no deleted_at condition and uses the index on created_at
 	// alone. The other tables keep the unindexed BaseModel created_at.
-	CreatedAt time.Time      `gorm:"index;index:idx_events_deleted_at_created_at,priority:2" json:"createdAt"`
-	DeletedAt gorm.DeletedAt `gorm:"index:idx_events_deleted_at_created_at,priority:1"       json:"deletedAt,omitzero"`
+	// DeletedAt is also the second column of the resubmitted_from_id
+	// index, for the reason DeliveryResult gives.
+	CreatedAt time.Time      `gorm:"index;index:idx_events_deleted_at_created_at,priority:2"                                           json:"createdAt"`
+	DeletedAt gorm.DeletedAt `gorm:"index:idx_events_deleted_at_created_at,priority:1;index:idx_events_resubmitted_from_id,priority:2" json:"deletedAt,omitzero"`
 
 	WebhookID    string `gorm:"type:uuid;not null" json:"webhookId"`
 	EntrypointID string `gorm:"type:uuid;not null" json:"entrypointId"`
@@ -42,7 +44,7 @@ type Event struct {
 	// existed. It is not a foreign key: the source event can be
 	// reaped by retention while its copies remain, and the id is
 	// kept as the record of where the copy came from either way.
-	ResubmittedFromID *string `gorm:"type:uuid;index" json:"resubmittedFromId,omitempty"`
+	ResubmittedFromID *string `gorm:"type:uuid;index:idx_events_resubmitted_from_id,priority:1" json:"resubmittedFromId,omitempty"`
 
 	// Relations. No model marshals the record it belongs to, so
 	// Webhook and Entrypoint are left out of the JSON.