1 files changed, 59 insertions, 0 deletions
diff --git a/db/migrate/20210201140434_add_oldest_merge_requests_index_again.rb b/db/migrate/20210201140434_add_oldest_merge_requests_index_again.rb
new file mode 100644
index 00000000000..a3fed9e576a
--- /dev/null
+++ b/db/migrate/20210201140434_add_oldest_merge_requests_index_again.rb
@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+class AddOldestMergeRequestsIndexAgain < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::SchemaHelpers
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  DOWNTIME = false
+
+  INDEX = 'index_on_merge_requests_for_latest_diffs'
+
+  def up
+    execute "DROP INDEX CONCURRENTLY #{INDEX}" if invalid_index?
+
+    return if index_exists_by_name?('merge_requests', INDEX)
+
+    begin
+      disable_statement_timeout do
+        execute "CREATE INDEX CONCURRENTLY #{INDEX} ON merge_requests " \
+          'USING btree (target_project_id) INCLUDE (id, latest_merge_request_diff_id)'
+      end
+    rescue ActiveRecord::StatementInvalid => ex
+      # Due to https://github.com/lfittl/pg_query/issues/184, if the CREATE
+      # INDEX statement fails, we trigger a separate error due to the Gem not
+      # supporting the INCLUDE syntax.
+      #
+      # To work around this, we raise a custom error instead, as these won't
+      # have a query context injected.
+      raise "The index #{INDEX} couldn't be added: #{ex.message}"
+    end
+
+    create_comment(
+      'INDEX',
+      INDEX,
+      'Index used to efficiently obtain the oldest merge request for a commit SHA'
+    )
+  end
+
+  def down
+    return unless index_exists_by_name?('merge_requests', INDEX)
+
+    disable_statement_timeout do
+      execute "DROP INDEX CONCURRENTLY #{INDEX}"
+    end
+  end
+
+  def invalid_index?
+    result = execute(<<~SQL)
+      SELECT pg_class.relname
+      FROM pg_class, pg_index
+      WHERE pg_index.indisvalid = false
+      AND pg_index.indexrelid = pg_class.oid
+      AND pg_class.relname = '#{INDEX}';
+    SQL
+
+    result.values.any?
+  end
+end