As this is a performance patch it's easier to compare before and after the patch is applied.   1) Apply a small hack to expose what sql is going to the primary db: diff --git a/lib/dml/moodle_read_slave_trait.php b/lib/dml/moodle_read_slave_trait.php index 95d05a49845..4452dcce12d 100644 --- a/lib/dml/moodle_read_slave_trait.php +++ b/lib/dml/moodle_read_slave_trait.php @@ - 284 , 6 + 284 , 7 @@ trait moodle_read_slave_trait { $ this ->set_db_handle($ this ->dbhreadonly); return ; } +error_log($sql . ' found at ' . format_backtrace(debug_backtrace(), true )); $ this ->set_dbhwrite(); }   2) Tail the logs and load a page and confirm you can see all the sql reads and writes 3) Now setup a fake read replica in config.php, you can set the replica to use the same database as the main one, eg: $CFG->dbhost = 'localhost' ; ... $CFG->dboptions = array ( ... 'readonly' => [ 'instance' => 'localhost' , ], );  4) Load a page and confirm the log is now much reduce and now only shows just writes and a few selected queries to mdl_config, mdl_sessions and 2 cursors 5) Apply the patch 6) Reload and confirm that the two cursor queries or now missing, but everything still works fine  Rinse and repeat with various important pages and confirm that there are no extraneous cursors in use a) / home page b) /my/ dashboard c) a course page