query-performance

Deleting millions of rows in MySQL

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I recently found and fixed a bug in a site I was working on that resulted in millions of duplicate rows of data in a table that will be quite large even without them (still in the millions). I can easily find these duplicate rows and can run a single delete query to kill them all. The problem is that trying to delete this many rows in one shot locks up the table for a long time, which I would like to avoid if possible. The only ways I can see to get rid of these rows, without taking down the site (by locking up the table) are:

Why is a parallel top N sort apparently much more CPU efficient than a serial top N sort?

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I’m testing against SQL Server 2019 CU14. I have a pure row-mode query that selects the top 50 rows from a complicated view. The full query takes 25426 ms of CPU time at MAXDOP 1 and 19068 ms of CPU time at MAXDOP 2. I’m not surprised that the parallel query uses less CPU time overall. The parallel query is eligible for bitmap operators and the query plan is different in a few ways. However, I am surprised by a large reported difference in operator times for the top N sort.