`label_histogram` runs on every label keystroke and after every batch of
judgements is saved. On the reference library it cost 7.2-8.4 ms best-of-50
by catalog_bench (13 ms on a busy machine), to report that none of 23,500
images carried a label.
The join was the rating histogram's, with one thing worse: the index does
not carry `label`, so each probe went on to read the version's row.
SCAN i USING COVERING INDEX images_folder
SEARCH v USING INDEX versions_judgement (image_id=?) LEFT-JOIN
USE TEMP B-TREE FOR GROUP BY
It now takes the rating histogram's shape: only labelled default versions
are grouped, and the unlabelled slot is what is left of `judged_rows`.
SCAN versions USING INDEX versions_judgement
USE TEMP B-TREE FOR GROUP BY (the labelled rows only)
That pass still reads each default version's row for `label`, but in the
index's order, which follows the table's; a partial index on the labelled
rows would make it index-only, and was not worth a new index for the
remaining 1 ms. After: 1.7-2.0 ms, the same answer on the reference library,
and a test that compares it with the old join over the awkward states the
rating test uses (a second default's label counted, unknown codes and zero
folded into unlabelled).
dtourolle
73059f2656