The following bug has been logged on the website:
Bug reference: 14614
Logged by: Sean Johnston
Email address: sean(dot)johnston(at)edgeintelligence(dot)com
PostgreSQL version: 9.6.2
Operating system: Debian 7.11
Description:
STATEMENT: ( ( select 1,2,3 ) union ( select 4,5,6 order by 1,2 ) order by
1,2 ) except ( select 4,5,6 ) order by 1,2;
ERROR: no relation entry for relid 0
Removing the middle order by works:
STATEMENT: ( ( select 1,2,3 ) union ( select 4,5,6 order by 1,2 ) ) except
( select 4,5,6 ) order by 1,2;
?column? | ?column? | ?column?
----------+----------+----------
1 | 2 | 3
(1 row)