From the review comments in bug 1540669:

For PlacesDBUtils we could probably use PARTITION BY to update ALL the bookmarks:

WITH positions(id, pos, seq) AS (
SELECT id, position AS pos,(row_number() OVER (PARTITION BY parent ORDER BY position)) - 1 AS seq
FROM moz_bookmarks
)
UPDATE moz_bookmarks
SET position = (SELECT seq FROM positions WHERE positions.id = moz_bookmarks.id)
WHERE id IN ( SELECT id FROM positions WHERE seq <> pos);

Just note that using the row_number() window function we can likely fix all of the indexing problems we have around.

We should see if we can apply this to reorderChildren().

https://hg.mozilla.org/mozilla-central/rev/782a1acfd6fd