From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Dilger <hornschnorter@gmail.com>
cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        Adrien Nayrat <adrien.nayrat@dalibo.com>,
        Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] PATCH: multivariate histograms and MCV lists
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Comments: In-reply-to Mark Dilger <hornschnorter@gmail.com>
	message dated "Sat, 25 Nov 2017 17:17:44 -0800"
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Date: Sat, 25 Nov 2017 21:35:35 -0500
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Mark Dilger <hornschnorter@gmail.com> writes:
>> On Nov 25, 2017, at 3:33 PM, Tomas Vondra <tomas.vondra@2ndquadrant.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 use=
d.  For
> conditions that can happen through (mis)use by the user, ereport is appr=
opriate.

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