Validate not null external_diff_store

Add remove an index which was added only to facilitate populating NULL
values.

changelogs/unreleased/mk-enforce-not-null-external-diff-store-on-mr-diffs.yml

+---
+title: Validate not null external_diff_store field on merge_request_diffs to maintain
+  data integrity
+merge_request: 42045
+author:
+type: added

db/post_migrate/20200910175553_validate_not_null_external_diff_store_on_merge_request_diffs.rb

+# frozen_string_literal: true
+
+class ValidateNotNullExternalDiffStoreOnMergeRequestDiffs < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  # Remove index which was only added to fill external_diff_store
+  INDEX_NAME = 'index_merge_request_diffs_external_diff_store_is_null'
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    validate_not_null_constraint :merge_request_diffs, :external_diff_store
+
+    remove_concurrent_index_by_name :merge_request_diffs, INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :merge_request_diffs, :id, where: 'external_diff_store IS NULL', name: INDEX_NAME
+  end
+end

db/schema_migrations/20200910175553

+904e0a8623df766a1f385bbb3db8942d10c4a92354d8f5e3bc03a813337c5fa1
\ No newline at end of file

db/structure.sql

@@ -13216,7 +13216,8 @@ CREATE TABLE public.merge_request_diffs (
     external_diff character varying,
     external_diff_store integer DEFAULT 1,
     stored_externally boolean,
-    files_count smallint
+    files_count smallint,
+    CONSTRAINT check_93ee616ac9 CHECK ((external_diff_store IS NOT NULL))
 );
 
 CREATE SEQUENCE public.merge_request_diffs_id_seq
@@ -17941,9 +17942,6 @@ ALTER TABLE public.vulnerability_scanners
 ALTER TABLE public.packages_package_files
     ADD CONSTRAINT check_4c5e6bb0b3 CHECK ((file_store IS NOT NULL)) NOT VALID;
 
-ALTER TABLE public.merge_request_diffs
-    ADD CONSTRAINT check_93ee616ac9 CHECK ((external_diff_store IS NOT NULL)) NOT VALID;
-
 ALTER TABLE ONLY public.ci_build_needs
     ADD CONSTRAINT ci_build_needs_pkey PRIMARY KEY (id);
 
@@ -20262,8 +20260,6 @@ CREATE UNIQUE INDEX index_merge_request_diff_files_on_mr_diff_id_and_order ON pu
 
 CREATE INDEX index_merge_request_diffs_by_id_partial ON public.merge_request_diffs USING btree (id) WHERE ((files_count > 0) AND ((NOT stored_externally) OR (stored_externally IS NULL)));
 
-CREATE INDEX index_merge_request_diffs_external_diff_store_is_null ON public.merge_request_diffs USING btree (id) WHERE (external_diff_store IS NULL);
-
 CREATE INDEX index_merge_request_diffs_on_external_diff_store ON public.merge_request_diffs USING btree (external_diff_store);
 
 CREATE INDEX index_merge_request_diffs_on_merge_request_id_and_id ON public.merge_request_diffs USING btree (merge_request_id, id);