Date: Thu, 20 Jun 2019 16:45:05 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: keith.fiske@crunchydata.com, pgsql-bugs@lists.postgresql.org,
	Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: BUG #15865: ALTER TABLE statements causing "relation already
 exists" errors when some indexes exist
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On 2019-Jun-20, PG Bug reporting form wrote:

> When testing the setup of our monitoring platform, we started running into
> an error when using PostgreSQL as a backend for Grafana. We narrowed down
> the issue to only occurring with the latest point release of PG,
> specifically 11.4, 10.9 and 9.6.14 (previous major versions were not tested
> at this time). The issue can be recreated by following the setup steps for
> Grafana with a PG backend and it will occur when the Grafana service is
> started for the first time and it tries to set up its schema in PG. We do
> not see the error occurring with the previous minor versions (11.3, 10.8,
> 9.6.13).

Confirmed.  Bisection says that

commit e76de886157b7f974d4d247908b242607cfbf043
Author:     Tom Lane <tgl@sss.pgh.pa.us>
AuthorDate: Wed Jun 12 12:29:24 2019 -0400
CommitDate: Wed Jun 12 12:29:39 2019 -0400

    Fix ALTER COLUMN TYPE failure with a partial exclusion constraint.

is the culprit.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services