Somehow tags with hundreds of questions were incorrectly synonymized and merged. I wrote a post on a local meta about this and a moderator unsynonymized the wrong tag. Now the tag that used to be false "main" tag has all questions of both tags and the other tag has no questions. Is there any reliable and fast way to restore the state of question tags to the same as before this incorrect synonymization?

Moderators on the local site don't know how to resolve this efficiently. Their suggestion, to try to distinguish questions by their content using SEDE queries and manually edit question tags, seems to me a slow and unreliable way of solving this issue.

Maybe there are some moderator tools they are missing or this can be resolved by request to Stack Exchange developers?

Specific details: I'm talking about angularjs (now has 1396 questions) and angular (0 questions) tags on Russian StackOverflow.

  • Synonymizing (remapping all new uses of a tag to another one) and merging (changing previously existing uses of a tag to another one) are two different steps. What's supposed to happen is that after the synonym gets created, there is supposed to be separate discussion as to whether or not the tags should be merged. This way, if the consensus is to not, the synonym can be removed without disruption. – Sonic the Anonymous Hedgehog Mar 25 '18 at 21:32
  • @SonicWizard So you mean that the root of this issue is tag merging, not synonymization? – Vadim Ovchinnikov Mar 25 '18 at 21:34
  • the tags were merged, which is an irreversible action. There's no way for moderators to undo that efficiently. – Mad Scientist Mar 25 '18 at 21:35
  • Yes, that's correct. The mods shouldn't have merged immediately upon the initial request to synonymize; they should have instead held another discussion to see if they should be merged. – Sonic the Anonymous Hedgehog Mar 25 '18 at 21:35
  • @SonicWizard Merging immediately is usually not an issue, because you should be damn sure anyway before merging tags. And on larger sites, you should never do this without a meta post anyway. I'd always merge when creating synonyms, it doesn't make much sense to only go half-way there. – Mad Scientist Mar 25 '18 at 21:37
  • @MadScientist This is the official advice given by the SE team. Also, for a tag this large (~1,400 questions), I personally think there should definitely have been a discussion. – Sonic the Anonymous Hedgehog Mar 25 '18 at 21:38
  • What a mess! I'm not sure if there's a way to reverse the merge process, but you can at least know (for the most part) which questions had the tag by looking at the data dump. – Laurel Mar 25 '18 at 23:58
  • @Laurel Too bad it's Sunday, or else we could also get them by SEDE. – Sonic the Anonymous Hedgehog Mar 26 '18 at 1:34

There is no undo for tag merges; the process doesn't just retag the questions involved, it re-writes the history of each post that used the losing tag to use the winner. When done, it is as though the tag was never used. This ensures the tag doesn't inadvertently creep back in during edits...

...But this also makes it really difficult to fix mistakes. In most cases, there's no choice but to go through and edit every question that needs the old tag, re-adding it as we go. In extreme cases, there's a tool that can be used by the Stack Overflow admins to re-add a tag to a set of questions in bulk - but note that this still doesn't leave the questions in the state they were in before, it merely automates the manual edit step.

In this case, the merge was done back in January of 2016; it's unlikely to be worthwhile trying to dig up data for which questions were tagged prior to that anyway, given how much time has passed. Better to establish a set of rules for using the tag and apply them now.

See also: What to do about Angular 2?

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