Re: [SQL] Howto to force NULL rows at the bottom ?

Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> Always test everything you say. The NULLs will come out at the end no
> matter which way you order it. There is a TODO item for this, but I
> suspect that the function manager clean up needs to be completed first,

I believe this is not an fmgr issue. The sort comparison routine (in
current sources, comparetup_heap() in backend/utils/sort/tuplesort.c)
checks for nulls before it calls the supplied comparison operator,
and it has a hard-wired decision that NULL sorts after non-NULL.
DESC order doesn't affect this at all (since that just chooses a
different operator).

To my mind, DESC doesn't have anything to do with whether NULLs sort
before or after non-NULLs; if we want a control for that, it should
be something else. It would be pretty easy to drive this off a SET
variable, if you didn't mind having it a global setting rather than
per-query.

The SQL spec seems to leave this up to the implementor:

Whether a sort key value that is null is considered greater
or less than a non-null value is implementation-defined, but
all sort key values that are null shall either be considered
greater than all non-null values or be considered less than
all non-null values.

regards, tom lane