# The list of related questions is shown empty

At the moment, any question that I check returns empty list of related questions. I see just a header called "Related" but no questions shown below that.

I have checked this on multiples sites and also while not logged in. (At the moment I do not have the possibility to try it from a different computre or from different IP - but maybe other will confirm whether they see the same thing, to check whether this is just my local problem.)

Here are some screenshots showing what the sidebar looks like. The first one is from Meta Stack Exchange:

From Mathematics Stack Exchange:

From MathOverflow (this site does not display HNQs in the sidebar):

• Might be one of Nick Cravers non-caching optimizations went south ... – rene Jul 9 at 8:22
• Seeing the same behaviour – Luuklag Jul 9 at 8:39
• A bug report on Mathematics Meta: No related questions are displayed. – Martin Jul 10 at 5:37
• @rene yeah, looks like the rule "Don't fix if it's not broken" isn't applied on SE. :/ – Shadow Wizard Wearing Mask Jul 13 at 13:17
• @ShadowWizardWearingMask well, I guess management looked at the SE meme page and asked: What is this blame caching thing? And then Nick was told to get rid of the meme / caching, whatever happens first. – rene Jul 13 at 13:33
• I'm confused as to why there is so little traction on this. The related questions list is crucial to find what one is looking for. Not having it is very annoying. I would have expected much more than 180 people (the current number of views of this question) to google what was going on in 5 days. Or is it only not working for some? – glS Jul 14 at 11:33
• This is still broken. :( IMHO it's very frustrating because it makes it much harder to search for good dupe targets. – PM 2Ring Jul 19 at 11:50
• Wouldn't related questions be the ones in the Linked list, and the linked ones the links people have put in their comments, or answers? – Ollie Jul 23 at 14:32
• @Ollie well sure, but the related questions list is what you can use to find the related questions to link – glS Jul 23 at 14:33
• @glS true...But why Linked first and Related second? – Ollie Jul 23 at 14:34
• The Linked list is for questions that have been explicitly linked on the page, the Related list is for questions that the system software automatically determines to be related. – PM 2Ring Jul 23 at 16:31
• @JourneymanGeek You have added the (status-review) tag. Was this done to bring this to the attention of Stack Exchange developers or as a sign to show that they are already looking at it. (AFAICT this tag is used both for escalation and as a signal that something is already in the process to be fixed. Although for the latter, the addition of the tag by some of the SE developers seems to be more likely.) – Martin Jul 24 at 6:49
• I don't work here... so the former ;) – Journeyman Geek Jul 24 at 6:54
• Hey all, just pinged on this. Y'all nailed it - this was me inadvertently beating the ever living crap out of SQL. We've fixed the issue that allowed it to runaway in the crazy case (we've hit this before, it's different here in that the failsafe mechanisms to render a sidebar blank and resume later didn't kick in...those have been fixed too). We've added additional monitoring we want to accumulate over the next few days and want to further cheapen the query (room to optimize) - after that, we'll re-activate the sidebar change and fix missing here. – Nick Craver Jul 24 at 17:22
• Thanks for the update @Nick, really not trivial to get one. :) – Shadow Wizard Wearing Mask Jul 24 at 17:45

As part of the sidebar changes for related questions, there were 2 issues here, which we didn't understand until today.

1. There was a SQL server overload (pegged at 100% for several minutes) when a query plan for the new sidebar went horribly wrong over 20 hours after deployment. A fix for this was deployed, but for a time period related questions weren't being rebuilt correctly and that includes some of those reported. This masked the second issue...
2. The change itself to the sidebar has an on/off mechanism. It's currently off while we do a few more things there. The on position was fine, but the off position was buggy. A property rename was missed which was piping the new property (silently, as a .ToString() on the collection of questions in Razor, without complaint) instead of the older StackHtmlString it should have been. This had the same symptoms as missing related questions from the database incident.

Number 2 has been fixed and I kicked off rebuilds for 1 (they should be completed by the time this is posted).

For anyone more curious about the technical bits of "on"/"off" here ("new"/"old"):

We went from caching full HTML strings for related questions (for 5 minutes - this was the "off" position) to querying it live every page load ("on"). Realistically, that cache is hit so little there's not a huge volume difference in queries. Only in hot or homepage questions is there a volume increase.

The tradeoff here is more often querying for a simplified cache, a lot less memory usage, and a generally healthier gen 2 state in our ecosystem (we're wrangling some not-compacting-as-much-as-we'd-like issues). In tuning this, we also found that:

• More columns are being brought back than we needed in the end views (for the sake of code reuse - but we can have some duplication for a page as critical as the question page to save a ton there)
• The queries could be pipelined - e.g. for collections (in Teams), linked, and related questions: we can throw those all at SQL at once for a question, eating one round-trip and read the result sets sequentially. This helps in higher latency environments and may be something we more generally do for this page overall.
• The ordering for Linked questions currently factors in views, answer count, and answer score. Due to how our Elastic indexing works w.r.t. changes to our Post table, we moved ViewCount off to another table many ages ago - so that's a join we can eliminate if we have a roughly similar sort for popularity: e.g. Score (of the question). It was fixed here at the time, but we could get more performance and lower overhead.

Overall, these are still changes we're going to make, but some bits external to this answer are in play on the timeline. We were close on having more monitoring metrics up in our SignalFX system, so I paused the deploy here (turning the flag back "on") while we got those metrics deployed (thanks Dean!), the dashboards setup, and some decent "before" recordings before we turn this on. We had all this data before in SQL to explore...but metrics makes it even easier to compare.

Anyway, that's a short rundown of what happened here and why. I hope it's helpful!

• Woohoo! I see Related Questions again! I never realized how much I relied on them for finding duplicates until they were gone... Thanks for adding the gory details. It was an interesting read. – ColleenV Jul 31 at 14:10
• Does this question on ELL really return no related questions, or is there possibly an issue? ell.stackexchange.com/questions/256879/… The tags have 100+ questions each and I have found some related questions that I've linked, and those show up fine. It seems odd that there isn't even a "Related" questions header. – ColleenV Aug 7 at 15:19
• Actually, it looks like there are a number of questions that have no "Related" questions section and I don't see a pattern yet. Here's another: ell.stackexchange.com/q/224841/9161 – ColleenV Aug 7 at 15:24