tag_summary table was introduced in 2009 as a roll up table for change_tag. One of the reasons it was being used was that MySQL databases that were using earlier versions of 4.1 (Released at 15 February 2005) could not use GROUP_CONCAT feature.

Around five years ago, developers started to replace usages of tag_summary with change_tag primarily because GROUP_CONCAT became available then and it most cases it was faster. For example [1] but it wasn't done fully which led us to having discrepancies. For example, Special:RecentChanges uses change_tag table but its API counterpart uses tag_summary table. Maintaining two extremely large tables is a technical debt that have been biting us since its deployment. Also, with normalization of change_tag table in place [2], it's more performant than tag_summary.

So we are replacing usages of this table with change_tag and in the next couple of weeks, and then we will drop the whole table. If you're using it in cloud replicas, please change it to change_tag. If you have any concerns or notes, feel free to chime in at https://phabricator.wikimedia.org/T209525 (Also, review of the patches would be extremely appreciated) 

Thank you and sorry for any inconvenience.

[1]: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/95584
[2]: https://phabricator.wikimedia.org/T185355

Amir Sarabadani
Software Engineer

Wikimedia Deutschland e. V. | Tempelhofer Ufer 23-24 | 10963 Berlin
Tel. (030) 219 158 26-0

Stellen Sie sich eine Welt vor, in der jeder Mensch an der Menge allen Wissens frei teilhaben kann. Helfen Sie uns dabei!

Wikimedia Deutschland – Gesellschaft zur Förderung Freien Wissens e. V. Eingetragen im Vereinsregister des Amtsgerichts Berlin-Charlottenburg unter der Nummer 23855 B. Als gemeinnützig anerkannt durch das Finanzamt für Körperschaften I Berlin, Steuernummer 27/029/42207.