UPDATE - which index follow?

I'd like to know from where-to where the UPDATE start and finish. Always from the lowest key to the high one? Or could update row in t开发者_运维百科he middle before update the first ones?

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It's usually some unknown function of the WHERE clause in the UPDATE but nothing is guaranteed, so don't count on anything, however...
You can add a order by clause to your update statement, like so:

UPDATE mytable SET mytable.myfield = mytable.id + 1 ORDER BY id;

Then the update statement will run from the lowest to the highest id.

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it is undefined. the entire transaction will be read consistent from the first update to the first commit.