https://bugzilla.wikimedia.org/show_bug.cgi?id=68918

--- Comment #1 from Sean Pringle <[email protected]> ---
I'm fairly sure this is a manifestation of the same bug in the github report,
which relates to TokuDB fractal indexes not (yet) using a bulk prefetching
optimization by design in a number of situations, including DELETE.

To prove this, convert the example DELETE to an equivalent SELECT and see the
runtime drop to <1s. That indicates to a possible work around using an
additional temp table and a subquery:

CREATE TEMPORARY TABLE CCats2 AS SELECT * FROM CCats;

DELETE cc FROM CCats cc JOIN (
   SELECT cc2.cat FROM CCats2 cc2
   JOIN commonswiki_p.page ON page_title = cc2.cat
   JOIN commonswiki_p.categorylinks ON cl_from = page_id
   WHERE cl_to = 'Hidden_categories' AND page_namespace = 14
) t ON cc.cat = t.cat;

Time allowing, I'll test TokuDB 7.1.8 before the panned September GA.

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