Re: Behavior of "OFFSET -1"

On Tue, Jul 22, 2014 at 12:49 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> 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?

Not I.

--
Robert Haas
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