percentile_cont(fraction) WITHIN GROUP (ORDER BY sort_expression)

percentile_disc(fraction) WITHIN GROUP (ORDER BY sort_expression) 

percentile_disc returns a value in your set/window, whereas percentile_cont will interpolate

SELECT	PERCENTILE_DISC (0.5) WITHIN GROUP (ORDER BY num)FROM	(		SELECT			*		FROM			generate_series (1, 10) num	) A

result: 5

SELECT	PERCENTILE_CONT (0.5) WITHIN GROUP (ORDER BY num)FROM	(		SELECT			*		FROM			generate_series (1, 10) num	) A

reslult: 5.5