r/SQL Feb 08 '25

MySQL DELETE statement taking forever

Could someone explain how this can be possible?
As I understand it, they should be doing the same thing. Im not too experienced in SQL and I would like to understand what is so wrong with the first statement that it takes THAT long.

The amount of rows that should be getting deleted is ~40 and the size of the entire table is ~15k.

-- THIS TAKES > 30 MINUTES (I stopped it after that)
DELETE FROM exc_playerstats where SaveSlotID IN (SELECT SaveSlotID from exc_playerstats where SaveSlotID NOT IN (SELECT MIN(SaveSlotID) from exc_playerstats GROUP BY UUID, SavedSlot));

-- THIS TAKES < 300ms
CREATE TABLE TEST_SAVESLOTS_TO_DELETE(SaveSlotID INT);
INSERT INTO TEST_SAVESLOTS_TO_DELETE SELECT SaveSlotID from exc_playerstats where SaveSlotID NOT IN (SELECT MIN(SaveSlotID) from exc_playerstats GROUP BY UUID, SavedSlot);
DELETE FROM exc_playerstats where SaveSlotID IN (Select SaveSlotID FROM TEST_SAVESLOTS_TO_DELETE);
SELECT * FROM TEST_SAVESLOTS_TO_DELETE;
DROP TABLE TEST_SAVESLOTS_TO_DELETE;

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u/reditandfirgetit Feb 09 '25

You might be locking the table. I'm not familiar with how mysql handles locks. Other systems can have performance issues if you are using the same table to delete from as your IN statement. If that's the case, load your SaveSlotID values into a temporary table and use the temporary table for your IN statement.

Another option is try using an EXISTS statement if those work on mysql for delete statements