MySQLdb is caching SELECT results?

Question

i'm running a SELECT query in a loop.

Once in a while, the database table is updated (by another program).

The first SELECT retrieves the correct data, but further calls in the loop return the first values.

How can I retrieve up-to-date data?

The only workaround I found so far, is reconnect to the DB on each iteration! In my example, uncommenting BOTH comment #1# and #2#. Uncommenting only #2# is not enough (i.e., cursor is recreated), the results are still cached.

Here's a working sample that gives the error.

import MySQLdb
from time import sleep

class DB:
    def __init__(self):
        self.connection = MySQLdb.connect(mysql_host, mysql_user, mysql_pass, mysql_db)
        self.cursor = self.connection.cursor()

    def get(self):
            sql = ''' SELECT id, message FROM mps_messages
                      WHERE topic=%s ORDER BY id LIMIT 1  '''
            #1# self.connect()
            #2# self.cursor = self.connection.cursor()
            self.cursor.execute(sql, ("topic",) )
            rec = self.cursor.fetchone()
            print rec

    def loop(self):
        while True:
            self.get()
            sleep(4)

db=DB()
db.loop()

• OS: ubuntu,
• python: 2.7.4
• mysqldb: 1.2.3
• mysql: 5.5.34

Answer 1

i had to add

connection.autocommit(True)

Adding SQL_NO_CACHE had no effect on the presented case, apparently because there was no caching involved.

I still don't understand why a SELECT needs COMMIT.

I'll open a new question about it.

Answer 2

You can use SELECT with SQL_NO_CACHE keyword:

SELECT SQL_NO_CACHE id, message 
FROM mps_messages
WHERE topic=%s 
ORDER BY id 
LIMIT 1 

Answer 3

Another solution to consider is change the table to a non transactional storage engine like MyISAM, if using transactions on that particular table is not compuslory