Add indexes to improve join table performance

Poor performance has been noted when deleting records, particularly
in the bulk-remove parents flow and when bulk-deleting vaccination
records.

A likely cause is that some join tables are only indexed in one
direction, which can make deletion queries slower.

Jira-Issue: MAV-6735

Author: jhenderson

app/models/class_imports_parent_relationship.rb

@@ -10,6 +10,7 @@
 # Indexes
 #
 #  idx_on_class_import_id_parent_relationship_id_8225058195  (class_import_id,parent_relationship_id) UNIQUE
+#  idx_on_parent_relationship_id_class_import_id_d7c05d6c2c  (parent_relationship_id,class_import_id) UNIQUE
 #
 # Foreign Keys
 #

app/models/cohort_imports_parent_relationship.rb

@@ -10,6 +10,7 @@
 # Indexes
 #
 #  idx_on_cohort_import_id_parent_relationship_id_c65e20d1f8  (cohort_import_id,parent_relationship_id) UNIQUE
+#  idx_on_parent_relationship_id_cohort_import_id_40fb9846d6  (parent_relationship_id,cohort_import_id) UNIQUE
 #
 # Foreign Keys
 #

db/migrate/20260428080729_add_reverse_indexes_to_import_join_tables.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+class AddReverseIndexesToImportJoinTables < ActiveRecord::Migration[8.1]
+  disable_ddl_transaction!
+
+  INDEXES = {
+    class_imports_parent_relationships: %i[
+      parent_relationship_id
+      class_import_id
+    ],
+    cohort_imports_parent_relationships: %i[
+      parent_relationship_id
+      cohort_import_id
+    ],
+    immunisation_imports_vaccination_records: %i[
+      vaccination_record_id
+      immunisation_import_id
+    ]
+  }.freeze
+
+  def change
+    INDEXES.each do |table_name, columns|
+      add_index table_name, columns, unique: true, algorithm: :concurrently
+    end
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_04_20_122440) do
+ActiveRecord::Schema[8.1].define(version: 2026_04_28_080729) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
   enable_extension "pg_trgm"
@@ -162,6 +162,7 @@
     t.bigint "class_import_id", null: false
     t.bigint "parent_relationship_id", null: false
     t.index ["class_import_id", "parent_relationship_id"], name: "idx_on_class_import_id_parent_relationship_id_8225058195", unique: true
+    t.index ["parent_relationship_id", "class_import_id"], name: "idx_on_parent_relationship_id_class_import_id_d7c05d6c2c", unique: true
   end
 
   create_table "class_imports_parents", id: false, force: :cascade do |t|
@@ -219,6 +220,7 @@
     t.bigint "cohort_import_id", null: false
     t.bigint "parent_relationship_id", null: false
     t.index ["cohort_import_id", "parent_relationship_id"], name: "idx_on_cohort_import_id_parent_relationship_id_c65e20d1f8", unique: true
+    t.index ["parent_relationship_id", "cohort_import_id"], name: "idx_on_parent_relationship_id_cohort_import_id_40fb9846d6", unique: true
   end
 
   create_table "cohort_imports_parents", id: false, force: :cascade do |t|
@@ -455,6 +457,7 @@
     t.bigint "immunisation_import_id", null: false
     t.bigint "vaccination_record_id", null: false
     t.index ["immunisation_import_id", "vaccination_record_id"], name: "idx_on_immunisation_import_id_vaccination_record_id_588e859772", unique: true
+    t.index ["vaccination_record_id", "immunisation_import_id"], name: "idx_on_vaccination_record_id_immunisation_import_id_813c516ad7", unique: true
   end
 
   create_table "important_notices", force: :cascade do |t|