Add primary key to mrcc_diff_files table

Add a migration to add a primary key on the
merge_request_context_commit_diff_files table on the columns
(merge_request_context_commit_id, relative_order).

changelogs/unreleased/270093-add-primary-key-to-mrcc-diff-files.yml

+---
+title: Add primary key to merge_request_context_commit_diff_files
+merge_request: 49024
+author:
+type: changed

db/migrate/20201202155913_add_primary_key_to_merge_request_context_commit_diff_files.rb

+# frozen_string_literal: true
+
+class AddPrimaryKeyToMergeRequestContextCommitDiffFiles < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    execute(<<~SQL)
+      DELETE FROM merge_request_context_commit_diff_files
+      WHERE merge_request_context_commit_id IS NULL;
+
+      DELETE FROM merge_request_context_commit_diff_files df1
+      USING merge_request_context_commit_diff_files df2
+      WHERE df1.ctid < df2.ctid
+        AND df1.merge_request_context_commit_id = df2.merge_request_context_commit_id
+        AND df1.relative_order = df2.relative_order;
+
+      ALTER TABLE merge_request_context_commit_diff_files
+      ADD CONSTRAINT merge_request_context_commit_diff_files_pkey PRIMARY KEY (merge_request_context_commit_id, relative_order);
+    SQL
+  end
+
+  def down
+    execute(<<~SQL)
+      ALTER TABLE merge_request_context_commit_diff_files
+      DROP CONSTRAINT merge_request_context_commit_diff_files_pkey,
+      ALTER COLUMN merge_request_context_commit_id DROP NOT NULL
+    SQL
+  end
+end

db/migrate/20201202161021_remove_redundant_index_on_merge_request_context_commit_diff_files.rb

+# frozen_string_literal: true
+
+class RemoveRedundantIndexOnMergeRequestContextCommitDiffFiles < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name :merge_request_context_commit_diff_files, 'idx_mr_cc_diff_files_on_mr_cc_id'
+  end
+
+  def down
+    # no-op: this index is not tracked in structure.sql, and is redundant due to idx_mr_cc_diff_files_on_mr_cc_id_and_sha
+  end
+end

db/schema_migrations/20201202155913

+926f54b5756fa9495e71f2340823418b5679195d5720212dddda0d0c6396629e
\ No newline at end of file

db/schema_migrations/20201202161021

+696c1d9f8cc90337549530e129e6abf4429d218f151cefaacacacb6e3f33d1c7
\ No newline at end of file

db/structure.sql

@@ -13640,7 +13640,7 @@ CREATE TABLE merge_request_context_commit_diff_files (
     old_path text NOT NULL,
     diff text,
     "binary" boolean,
-    merge_request_context_commit_id bigint
+    merge_request_context_commit_id bigint NOT NULL
 );
 
 CREATE TABLE merge_request_context_commits (
@@ -19473,6 +19473,9 @@ ALTER TABLE ONLY merge_request_blocks
 ALTER TABLE ONLY merge_request_cleanup_schedules
     ADD CONSTRAINT merge_request_cleanup_schedules_pkey PRIMARY KEY (merge_request_id);
 
+ALTER TABLE ONLY merge_request_context_commit_diff_files
+    ADD CONSTRAINT merge_request_context_commit_diff_files_pkey PRIMARY KEY (merge_request_context_commit_id, relative_order);
+
 ALTER TABLE ONLY merge_request_context_commits
     ADD CONSTRAINT merge_request_context_commits_pkey PRIMARY KEY (id);