Add a temporary index for Member.state

To temporarily improve the performance of an upcoming data migration we
need to add an index on the Member state column. This index can be
removed after 14.8

Changelog: added

db/migrate/20211223125921_add_temp_index_to_members_state.rb

+# frozen_string_literal: true
+
+class AddTempIndexToMembersState < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'tmp_index_members_on_state'
+
+  def up
+    # Temporary index to be removed in 14.9 https://gitlab.com/gitlab-org/gitlab/-/issues/349960
+    add_concurrent_index :members, :state, name: INDEX_NAME, where: 'state = 2'
+  end
+
+  def down
+    remove_concurrent_index_by_name :members, INDEX_NAME
+  end
+end

db/schema_migrations/20211223125921

+e08c1634376ed78b78c4a54d874bed66c1ce40c7c509b67cfda00d1a8657f127
\ No newline at end of file

db/structure.sql

@@ -28033,6 +28033,8 @@ CREATE INDEX tmp_idx_deduplicate_vulnerability_occurrences ON vulnerability_occu
 
 CREATE INDEX tmp_idx_vulnerability_occurrences_on_id_where_report_type_7_99 ON vulnerability_occurrences USING btree (id) WHERE (report_type = ANY (ARRAY[7, 99]));
 
+CREATE INDEX tmp_index_members_on_state ON members USING btree (state) WHERE (state = 2);
+
 CREATE INDEX tmp_index_namespaces_empty_traversal_ids_with_child_namespaces ON namespaces USING btree (id) WHERE ((parent_id IS NOT NULL) AND (traversal_ids = '{}'::integer[]));
 
 CREATE INDEX tmp_index_namespaces_empty_traversal_ids_with_root_namespaces ON namespaces USING btree (id) WHERE ((parent_id IS NULL) AND (traversal_ids = '{}'::integer[]));