Largish (but not too large) queries are failing with URI/header "too large" errors when using the interface at https://query.wikidata.org
Browser/OS: Chrome 72 and Firefox 65 on Windows 10.
<h1>Bad Message 431</h1><pre>reason: Request Header Fields Too Large</pre>
<html> <head><title>414 Request-URI Too Large</title></head> <body bgcolor="white"> <center><h1>414 Request-URI Too Large</h1></center> <hr><center>nginx/1.13.6</center> </body> </html>
Once the query grows sufficiently large it works fine again: example
I first noticed this around the 9th of January (2019) and posted here: https://www.mediawiki.org/wiki/Talk:Wikidata_Query_Service#Error:_%22414_Request-URI_Too_Large%22
As that post states, I had an unchanged query that was working only a couple of days before that.
Technically, I believe that those errors arise due to the limits on GET requests, as noted in the manual. But if you look at the working example with the network tab open in your browser devtools, the GUI can transparently switch/fallback to making a POST request.
Looking at the code repo for the client, the frontend logic that controls this fallback doesn't seem to have changed anytime recently, so my assumption is that there has been some sort of server upgrade or configuration change that changed how it responds when it fails. I'm guessing the frontend needs to be changed to account for that.