Thread: Re-run query on automatic reconnect
Is there any particular reason why psql doesn't re-run a query that failed due to disconnect from the server after re-connecting? I've thought maybe it's because it might somehow be dangerous, but I can't think of any case where that's actually true. -- Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
On Mon, Sep 26, 2005 at 01:23:03PM -0500, Jim C. Nasby wrote: > Is there any particular reason why psql doesn't re-run a query that > failed due to disconnect from the server after re-connecting? I've > thought maybe it's because it might somehow be dangerous, but I can't > think of any case where that's actually true. What if the query itself resulted in the disconnect by causing the backend to crash? Re-running such a query automatically would be a bad idea. Or did I misunderstand what you're asking? -- Michael Fuhr
"Jim C. Nasby" <jnasby@pervasive.com> writes:
> Is there any particular reason why psql doesn't re-run a query that
> failed due to disconnect from the server after re-connecting? I've
> thought maybe it's because it might somehow be dangerous, but I can't
> think of any case where that's actually true.
You haven't thought very hard.
(1) psql cannot tell whether the query was already completed before the
connection dropped; if so, reissuing it would make its effects if any
happen twice (eg, duplicate insert, or adding something to a bank
balance twice).
(2) If inside a transaction block, the query might or might not depend
on previous operations in the same transaction.
(3) The query might depend on previous session-local operations,
such as SET commands or creation of temp tables.
(4) If the query actually caused the server crash, re-issuing it will
probably cause another crash. Instant infinite loop, complete with
denial of service to all other database users.
regards, tom lane
On Mon, Sep 26, 2005 at 02:52:02PM -0600, Michael Fuhr wrote: > On Mon, Sep 26, 2005 at 01:23:03PM -0500, Jim C. Nasby wrote: > > Is there any particular reason why psql doesn't re-run a query that > > failed due to disconnect from the server after re-connecting? I've > > thought maybe it's because it might somehow be dangerous, but I can't > > think of any case where that's actually true. > > What if the query itself resulted in the disconnect by causing the > backend to crash? Re-running such a query automatically would be > a bad idea. Or did I misunderstand what you're asking? The case I was thinking of was... psql> run some command... psql returns from call user does something else; meanwhile, server forced shutdown/crash user comes back, tries to run some query and gets automatic reconnect In any case, Tom brought up a bunch of cases where this would be a bad idea, so my question's answored. -- Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461