php - What is a proper length of random code used as mysql select / join attribute -

i assign each mysql row random code. alphanumeric lowercase characters , numbers used.

these random codes used select , join queries.

most use length of 24 characters gives 2.2x10^37 available combinations.

what perfect random code length not require checking duplicates while being no collisions occur keeping query result time minimum?

note: on big tables use index random code column of length 24.

the nature of random code can never not duplicated. can guarantee there wont duplicate if use sequential number & if doing might use auto increment option sufficiently large integer.

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