Hello,

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

Best

--

Amir Sarabadani

Software Engineer

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

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