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Why would the second way be faster? Generally speaking, the first form will perform worse (as well as looking a lot worse) than the second. You are hitting an edge case where the opposite is true...
Answer
#1: Initial revision
by
Jack Douglas
·
2020-08-09T17:52:27Z (over 4 years ago)
> Why would the second way be faster? Generally speaking, the first form will perform worse (as well as looking a lot worse) than the second. You are hitting an edge case where the opposite is true, because: 1. The `not in` in your first example is likely to be transformed into an anti-join (something like [this](https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=4436e351646efbb4a99708f09ef9904f)). Because you also have "…just a handful of records where a bit(1) column is set to 1…" that anti-join is likely to be fairly fast. 2. Bad stats or bad luck means that the optimizer is making a wrong choice when filtering. Perhaps it is choosing a full table scan in the second case, or failing to use a good index. We'd need to know your actual plans/indexes/etc to be able to say more, as several people have mentioned in comments.