more accurately detect if schema cache needs cleared

and add override to force-clear it

Change-Id: I05a4b68be963ca8eceace456cc2d81f4556a9da6
Reviewed-on: https://gerrit.instructure.com/160992
Tested-by: Jenkins
Reviewed-by: James Williams  <jamesw@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

lib/tasks/db.rake

@@ -2,7 +2,26 @@
 # not once per shard, and after the migrations have run
 %w{db:migrate db:migrate:predeploy db:migrate:up db:migrate:down db:rollback}.each do |task_name|
   Rake::Task[task_name].enhance do
-    ActiveRecord::Base.connection.tables.each do |table_name|
+    def sanitize_sequences(columns)
+      columns.map do |details|
+        details[2] = 'sequence' if details[2]&.start_with?('nextval')
+        details
+      end
+    end
+
+    connection = ActiveRecord::Base.connection
+    connection.tables.each do |table_name|
+      unless ENV['CLEAR_SCHEMA_CACHE']
+        cached_value = sanitize_sequences(connection.column_definitions(table_name))
+        ActiveRecord::Base.in_migration = true
+        actual_value =
+          begin
+            sanitize_sequences(connection.column_definitions(table_name))
+          ensure
+            ActiveRecord::Base.in_migration = false
+          end
+        next if cached_value == actual_value
+      end
       MultiCache.delete(["schema", table_name])
     end
   end