I've tracked down a weird MySQL problem to the two different ways I was performing a query. When you boil everything down, this way returns more results:
SELECT DISTINCT <stuff> FROM <tables>
WHERE promo_detail_store_id in (8214, 8217, 4952, 8194, ...)
This change to the WHERE clause produces a subset of those results:
WHERE promo_detail_store_id in ('8214, 8217, 4952, 8194, ...')
(promo_detail_store_id is defined as a BIGINT in a MyISAM table.)
Originally that list of store_ids was much longer, and I started cutting it shorter and shorter thinking maybe there was some weird limits on the length of a string. But no, it holds for quite small strings/lists too. Clearly something is going on behind the scenes involving type coercion and maybe how the 'in' operator works. Can someone enlighten me?
WHERE promo_detail_store_id in (8214, 8217, 4952, 8194, ...)
means
WHERE promo_detail_store_id = 8214
OR promo_detail_store_id = 8217
OR promo_detail_store_id = 4952
OR promo_detail_store_id = 8194
OR ...
WHERE promo_detail_store_id in ('8214, 8217, 4952, 8194, ...')
means
WHERE promo_detail_store_id = '8214, 8217, 4952, 8194, ...'
'8214, 8217, 4952, 8194, ...' will cast to number to be 8214, so it will be
WHERE promo_detail_store_id = 8214