Behavior of "OFFSET -1"

Before 9.3, you got an error from this:

regression=# select * from tenk1 offset -1;
ERROR: OFFSET must not be negative

But 9.3 and up ignore the negative OFFSET. This seems to be a thinko in
my commit 1a1832eb. limit_needed() thinks it can discard the Limit plan
node altogether, which of course prevents nodeLimit.c from complaining:

/* Executor would treat less-than-zero same as zero */
if (offset > 0)
return true; /* OFFSET with a positive value */

I don't recall the reasoning behind that comment for sure, but I imagine
I examined the behavior of ExecLimit() and failed to notice that there
was an error check in recompute_limits().

This seems to me to be a clear bug: we should reinstate the former
behavior by tightening this check so it only discards OFFSET with a
constant value of exactly 0. Anyone think differently?

regards, tom lane