Обсуждение: 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)
returntrue; /* 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
On Tue, Jul 22, 2014 at 12:49 PM, Tom Lane <tgl@sss.pgh.pa.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 EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
On Tue, Jul 22, 2014 at 12:49:37PM -0400, Tom Lane wrote: > Before 9.3, you got an error from this: > > regression=# select * from tenk1 offset -1; > ERROR: OFFSET must not be negative That seems eminently sane, and should continue to error out, IM. The only circumstance I can imagine where this could be argued not to be is just casuistry, namely LIMIT m OFFSET -n might be argued to mean LIMIT m-n. Cheers, David. -- David Fetter <david@fetter.org> http://fetter.org/ Phone: +1 415 235 3778 AIM: dfetter666 Yahoo!: dfetter Skype: davidfetter XMPP: david.fetter@gmail.com iCal: webcal://www.tripit.com/feed/ical/people/david74/tripit.ics Remember to vote! Consider donating to Postgres: http://www.postgresql.org/about/donate