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Re: Select random rows when using UUID
Posted by: Rick James
Date: June 18, 2016 01:35AM

RIGHT( HEX( (1<<24) * (1+RAND()) ), 6)

That finds a random starting point within the table, assuming that UUID is indexed.

1<<24 is 2**24, which is 1048576.
1+RAND() is a number between 1 and 2.
The product of those is a number between 1048576 and 2097152.
The HEX of that would be a 7-digit hex number.
RIGHT(,6) gets the bottom 6 -- random and a full 6 hex digits, possibly with leading zeros.

This would have been simpler, but then there would be trouble with not getting leading zeros:
HEX( (1<<24) * RAND()

WHERE HEX(LEFT(y.id, 6)) > init.start
   OR HEX(LEFT(y.id, 6)) < init.start -- why do you have this?

One rule in optimization is to not hide an indexed column (y.id) inside a function (HEX(LEFT(...))). So, this might be better:
WHERE y.id > HEX(init.start)

But that again gets in trouble with leading zeros.

Here's another way to deal with such:
RIGHT(CONCAT('000000', init.start), 6)
Better yet would be to fold that into init.

But...
FLOOR(RAND() * (@max - @min) + @min)
is useful only for consecutive value, such as with AUTO_INCREMENT, not with UUIDs.

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Re: Select random rows when using UUID
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June 18, 2016 01:35AM


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