postgresql - Why does calling random() inside of a case statement produce unexpected results? -

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https://gist.github.com/anonymous/2463d5a8ee2849a6e1f5

query 1 not produce expected results. however, queries 2 , 3 do. why moving call random() outside of case statement matter?

consider first expression:

select (case when round(random()*999999) + 1 between 000001 , 400000 1              when round(random()*999999) + 1 between 400001 , 999998 2              when round(random()*999999) + 1 between 999999 , 999999 3              else 4         end) generate_series(1, 8000000)

presumably, thinking value "4" should never selected. but, problem random() being called separately each when clause.

so, chance fails each clause independent:

  • about 60% of time random number not match "1".
  • about 40% of time random number not match "2".
  • about 99.9999% of time random number not match "3" (i apologize if number of nines off, value practically 1).

that means 24% of time (60% * 40% * 99.9999%), value "4" appear. in actual fact, first query returns "4" 23.98% of time. honest, close actual value, given size of data, bit further off expect. however, close enough explain happening.


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