Anomie added a comment.

  Using tags has the advantage of more directly identifying the relevant 
revisions, //if// the planner decides that gathering all revisions with the tag 
then filtering by which are in RC (and probably filesorting) is a better plan 
than taking rows from RC (in order) and filtering by which have the tag. The 
equivalent querying the slots table is much less likely to be workable since it 
would have to fetch all revisions with the slot rather than all revisions 
actually changing the slot. On the other hand, I'm skeptical that long-term 
there would be few enough revisions so tagged that the tag-first plan would 
actually be better.
  
  If the planner is going with an RC-first plan, then it seems unlikely to 
matter much whether it filters by joining `change_tag` or joining `slots`. 
There's no always-good option here, and the maybe-bad parts are the same for 
both.
  
  Personally I'd lean against adding lots of rows to `change_tag`, which will 
be visible in various UIs, if the only use case is filtering on slots edited. 
Using a wider index (bigint+bigint+smallint rather than big 
<https://phabricator.wikimedia.org/T63111>​int+int) for a filtering-join seems 
less troublesome than adding more to an already-tall table. But you might want 
to ask the DBAs for their opinion.

TASK DETAIL
  https://phabricator.wikimedia.org/T230862

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