`adopt_orphan_terms` runs in the backfill on every catalog open. Its
check -- is there a vocabulary row for this word, tombstones included --
cannot use `keyword_terms_name`, which is partial on `deleted = 0`, so the
correlated subquery scanned the vocabulary once for each of the 10,800
assignment rows before `DISTINCT` threw the repeats away: 3.5 ms per open
on the reference library.
The distinct words are taken first and the check runs once per word -- a
few dozen scans of a few dozen rows. Same rows out, since `DISTINCT` over
the assignments is exactly the set of words.
catalog_bench, best of 20: 3.45 ms -> 0.30 ms.
dtourolle
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