Moodle中不存在表'mdl_sessions'

Question

my moodle site display 'error reading from database'
after activating debugging in config.php error showed that table with name 'mdl_session' not exist in database

the error message is:

Debug info: Table 'mniknami_edu.mdl_sessions' doesn't exist
SELECT id, sid, state, userid, lastip, timecreated, timemodified FROM mdl_sessions     WHERE sid = ?
[array (
0 => 'lhdasq4iscedtqhvbs5j1lq833',
)]
Error code: dmlreadexception
Stack trace:    

line 441 of /lib/dml/moodle_database.php: dml_read_exception thrown
line 996 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
line 1415 of /lib/dml/moodle_database.php: call to mysqli_native_moodle_database->get_records_sql()
line 1387 of /lib/dml/moodle_database.php: call to moodle_database->get_record_sql()
line 1366 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()
line 268 of /lib/classes/session/manager.php: call to moodle_database->get_record()
line 79 of /lib/classes/session/manager.php: call to core\session\manager::initialise_user_session()
line 806 of /lib/setup.php: call to core\session\manager::start()
line 35 of /config.php: call to require_once()
line 27 of /login/index.php: call to require()

my moodle work correctly 2 days ago and Since then, I did not make any changes or update to it
why this problem arise and how i can fixed it?
my moodle version is 2.6.1+

Answer 1

Moodle sessions can be stored in differents places : database, files, memcached. According to https://docs.moodle.org/en/Session_handling the "[files] driver is used by default in new installation".

Perhaps that was the case, but something was recently changed in config.php and Moodle is now trying to use the database for sessions?

On a sidenote, using memcached for session makes a huge difference in performance (as said on Moodle's wiki), but the configuration needs to be done in config.php. If you have memcached available, this could fix the problem and make your site faster.