Hi List, Note beforehand: this question is a result of a stack-exchange that can be seen here: http://stackoverflow.com/questions/39624241/inconsistent-behaviour-of-set-returning-functions-in-sub-query-with-random
I'm often using the WHERE clause random() > 0.5 to pick a random subset of my data. Now I noticed that when using a set-returning function in a sub-query, I either get the whole set or none (meaning that the WHERE random() > 0.5 clause is interpreted *before* the set is being generated). e.g.: SELECT num FROM ( SELECT unnest(Array[1,2,3,4,5,6,7,8,9,10]) num) AS foo WHERE random() > 0.5; This seems inconsistent because the following query *does* take the whole set into account: SELECT num FROM ( SELECT * FROM unnest(Array[1,2,3,4,5,6,7,8,9,10]) num) AS foo WHERE random() > 0.5; So does this one: WITH foo AS ( SELECT unnest(Array[1,2,3,4,5,6,7,8,9,10]) num) SELECT num FROM foo WHERE random() > 0.5; Could anyone reflect on the seeming inconsistency here? I do understand that the planner sees the queries quite different (as can be seen from an EXPLAIN) but I don't understand the rationale behind it. Notes: - couldn't find another function to test apart from random(), but likely there is some - I tested with generate_series and as well - My real use case works with postgis and pgpointcloud where a range of set-returning functions is used in this manner Thanks, Tom