order by

I keep on investigating on the "death postgres" subject

but open a new thread as I don't know if it's related to my pb.

I have 2 different clusters, on 2 different machines, one is prod, the second test.

Same data volumes.

On prod if I do  

select col_a, count(col_a) from table_a group by col_a order by col_a desc, 

I get the numbers of NULL on top.

To get the number of NULL on top on the test db, I have to 

select col_a, count(col_a) from table_a group by col_a order by col_a asc.

so, it looks like there is something different within the b-tree operator class of varchar (?!?)

between those 2 clusters.

What can I check to to explain this difference as, to my understanding, it's not a postgresql.conf parameter.

thanks

Marc MILLAS