perf, add index and transaction around create campaign

Signed-off-by: RonniSkansing <rskansing@gmail.com>

backend/database/recipientGroupRecipient.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"github.com/google/uuid"
+	"gorm.io/gorm"
 )
 
 const (
@@ -20,3 +21,11 @@ type RecipientGroupRecipient struct {
 func (RecipientGroupRecipient) TableName() string {
 	return RECIPIENT_GROUP_RECIPIENT_TABLE
 }
+
+// Migrate adds an index leading with recipient_group_id. The existing unique index
+// leads with recipient_id, so counting or listing a group's members
+// (WHERE recipient_group_id = ?) could not use it and scanned the recipients per
+// group, making the group list quadratic in recipients times groups.
+func (RecipientGroupRecipient) Migrate(db *gorm.DB) error {
+	return db.Exec(`CREATE INDEX IF NOT EXISTS idx_rgr_group ON recipient_group_recipients(recipient_group_id, recipient_id)`).Error
+}