How do work tercile, percentile & funcion percentile_cont() ?

Hello,

From a table x(c1) containing 30 lines with integer values (column c1) from 1 to 30 :

SELECT percentile_cont(1./3) WITHIN GROUP (ORDER BY c1) FROM x ;
percentile_cont
------------------
10.6666666666667
(1 ligne)
SELECT percentile_cont(2./3) WITHIN GROUP (ORDER BY c1) FROM x ;
percentile_cont
------------------
20.3333333333333
(1 ligne)

If ordering,
the first 10 lines (c1 from 1 to 10) have ntile(3) OVER(ORDER BY c1) = 1,
the 10 following lines (c1 from 11 to 20) have ntile(3) OVER(ORDER BY c1) = 2,
the last 10 lines (c1 from 21 to 30) have ntile(3) OVER(ORDER BY c1) = 3.
So, I though it should be :
percentile_cont(1./3) = 10.5 (the arithmetic average between 10 et 11) and not 10.6666666666667
percentile_cont(2./3) = 20.5 (the arithmetic average between 20 et 21) and not 20.3333333333333

Thank in advance for explanation

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PALAYRET JACQUES
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