Re: [HACKERS] PATCH: multivariate histograms and MCV lists

Mark Dilger <hornschnorter(at)gmail(dot)com> writes:
>> On Nov 25, 2017, at 3:33 PM, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> I might be missing something, but why would ereport be more appropriate
>> than elog? Ultimately, there's not much difference between elog(ERROR)
>> and ereport(ERROR) - both will cause a failure.

The core technical differences are (1) an ereport message is exposed for
translation, normally, while an elog is not; and (2) with ereport you can
set the errcode, whereas with elog it's always going to be XX000
(ERRCODE_INTERNAL_ERROR).

> I understand project policy to allow elog for error conditions that will be reported
> in "can't happen" type situations, similar to how an Assert would be used. For
> conditions that can happen through (mis)use by the user, ereport is appropriate.

The project policy about this is basically that elog should only be used
for things that are legitimately "internal errors", ie not user-facing.
If there's a deterministic way for a user to trigger the error, or if
it can reasonably be expected to occur during normal operation, it should
definitely have an ereport (and a non-default errcode).

regards, tom lane