db/sqlachemy/session.py resets the mysql session type on every operation

[Doug Hellmann]

The callback to set the session type for mysql (http://git.openstack.org/cgit/openstack/oslo-incubator/tree/openstack/common/db/sqlalchemy/session.py#n524) is registered for the "checkout" event (http://git.openstack.org/cgit/openstack/oslo-incubator/tree/openstack/common/db/sqlalchemy/session.py#n635) which causes it to run every time a connection is retrieved from the pool. That means when a connection is reused, we update the session type.

Is that necessary? How expensive is it? Can we do it one time per connection, or one time per session?

See also https://bugs.launchpad.net/oslo/+bug/1288443

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/78628

[Roman Podoliaka (rpodolyaka)]

So yeah, currently it is executed once per session (and effectively once per database transaction). But MySQL 'session' variables have a client connection scope, so it's indeed not necessary to set sql_mode variable every time we start a new transaction. Though, I don't think it's any expensive.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/78628
Committed: https://git.openstack.org/cgit/openstack/oslo-incubator/commit/?id=d1988b96266cf275fdb2c2cee01adc46797004fb
Submitter: Jenkins
Branch: master

commit d1988b96266cf275fdb2c2cee01adc46797004fb
Author: Ben Nemec <email address hidden>
Date: Wed Mar 12 16:01:26 2014 +0000

    Set sql_mode callback on connect instead of checkout

    There's no need to set the sql_mode on every checkout of a
    connection - once when the connection is created is sufficient.

    Change-Id: If34db31d9646c9a75d591e556a7abcc0bf8a7ab1
    Closes-Bug: #1288444