Manual:$wgMiserMode

Miser mode is a mode intended to reduce load on server farms running a large number of wikis. It is enabled on all WMF wikis.

Details
If, disable database-intensive features, so that they can be managed/controlled separately if desired. This includes reading special pages marked as expensive from the cache instead of regenerating them every time they are requested.

With  set to , this means that the special pages managed by this flag will only be updated when explicitly made to update. For example, setting up a cron-job to call  or using.

The other disabled features are...


 * Disables custom formatting of change sizes in Special:RecentChanges via MediaWiki:Rc-change-size


 * Disables selecting all pages that start with x box on Special:Log (and  option of   API module)


 * Totally disables Special:MimeSearch, as well as  and   option in the   and   API module


 * Disables  option of the RSS feed for Special:Contributions


 * In the  and   API module, use reduced sorting by namespace mode (returns only a few results when   or   option is in use).


 * Similarly disables the filter by namespace box on Special:LinkSearch


 * Doesn't update number of active users quite on Special:Statistics as often (?)


 * Disables the search for images with x somewhere in their name box on Special:NewImages and Special:ListFiles


 * Do not show how many previous versions of an image were uploaded on Special:ListFiles


 * When running  maintenance script, will not re-auto-patrol edits by users with auto-patrol flag.


 * When running  maintenance script, will not tell you how many rows in total there are to update.


 * Disables transclusion count on action as of 109710.


 * Disables the magic word in content namespaces as defined with  as of 294774 .  It is substituted by a single dash after saving the page.


 * Disables the magic word in subject namespaces as of 570985 .  It is substituted by a single dash after saving the page.

For reference, this very wiki uses miser mode. You can tell if a wiki has miser mode enabled via the API module.

update.php
This configuration option disables in 1.18.0 (and only 1.18.0. Subsequent releases will not use this option for that purpose), requiring one to use  to run that script. Very very very large wikis (like Wikipedia) may need to have more careful upgrade process than the maintenance script does, but most sites would not have to worry about this.