`rating_histogram` runs on every star keystroke. On the reference library
(24k images, 1,200 of them rated) it cost 6.3 ms best-of-50 by
catalog_bench, and up to 10-14 ms when the machine is busy.
It was `images LEFT JOIN versions ON ... AND is_default = 1 GROUP BY
rating`. The plan:
SCAN i USING COVERING INDEX images_folder
SEARCH v USING COVERING INDEX versions_judgement (image_id=?) LEFT-JOIN
USE TEMP B-TREE FOR GROUP BY
A probe of the index per image, then a sort of all 23,500 rows, to put
22,000 of them in slot zero.
Now the rated rows are grouped on their own (`rating != 0`: one pass over
`versions_judgement`, a sort of 1,200 rows), and slot zero is what is left
of the join's row count. That count is three index-only aggregates -- the
library size, the default versions, and the images holding one -- so an
image with no version is still unrated, and an image with two default
versions still counts twice, exactly as the join counted it:
SCAN versions USING COVERING INDEX versions_judgement (x3)
SCAN images USING COVERING INDEX images_folder
`count(DISTINCT image_id)` has its own statement because alone it reads the
distinct values off the index order; beside other aggregates SQLite builds a
temporary b-tree for it.
After: 1.2 ms. The histogram is the same on the reference library
([22364, 663, 19, 47, 115, 374]), and a new test compares it with the old
join on a catalog holding every state the schema allows: no version, only a
virtual copy, two defaults, ratings below zero and above five.
dtourolle
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