Add unique index constraint to segmented users (#19627)

* Add unique index constraint to segmented users

* Run migration

* Add de-duping query

db/migrate/20230622151208_remove_existing_segmented_users.rb

@@ -0,0 +1,18 @@
+class RemoveExistingSegmentedUsers < ActiveRecord::Migration[7.0]
+  def up
+    sql = <<-SQL
+      DELETE FROM segmented_users deduped_seg_users
+      USING segmented_users current_seg_users
+      WHERE
+        deduped_seg_users.id < current_seg_users.id AND
+        deduped_seg_users.user_id = current_seg_users.user_id AND
+        deduped_seg_users.audience_segment_id = current_seg_users.audience_segment_id;
+    SQL
+
+    ActiveRecord::Base.connection.execute(sql)
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20230622151553_add_unique_constraint_to_segmented_users.rb

@@ -0,0 +1,26 @@
+class AddUniqueConstraintToSegmentedUsers < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
+  def up
+    # remove the old index first
+    remove_index :segmented_users, name: "index_segmented_users_on_audience_segment_and_user", algorithm: :concurrently
+
+    # add a new index with uniqueness constraint
+    add_index :segmented_users,
+              %i[audience_segment_id user_id],
+              unique: true,
+              name: "index_segmented_users_on_audience_segment_and_user",
+              algorithm: :concurrently
+  end
+
+  def down
+    # to reverse the migration, remove the unique index
+    remove_index :segmented_users, name: "index_segmented_users_on_audience_segment_and_user", algorithm: :concurrently
+
+    # and add back the old index
+    add_index :segmented_users,
+              %i[audience_segment_id user_id],
+              name: "index_segmented_users_on_audience_segment_and_user",
+              algorithm: :concurrently
+  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[7.0].define(version: 2023_06_08_154310) do
+ActiveRecord::Schema[7.0].define(version: 2023_06_22_151553) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "citext"
   enable_extension "pg_trgm"