Yeah, get back to me when you have data…

User: “Hey, my SQL queries are timing out on the replica!”

J: “Hmmm, that error says they’re so slow that the master is trying to clean up all the rows that have changed since it started running.”

U: “Can we try it on the primary?”

J: “No, because 1) Production, and 2) error is specific to replicas.”

U: (CCs Partner)

Partner: “Our service that queries your DB, sends the results over a VPN tunnel, and ingests them into our system is working fine, and doesn’t show any delays or network issues.”

J: “That’s what you said last time, and the problem ‘just went away’ the next night.”

P: “Try running this specific query locally and tell me how it works.”

J: (examines 450-line SQL query, shrugs, runs it) “2.5 minutes.”

P: “Hmm, works here with ‘limit 10000’ and fails with ‘limit 50000’. Well, nothing we can do on our end! Shall I set up a call with our Engineering team?”

J: “Wait, what was the runtime for that query on your end before it started timing out this week? For that matter, what was the runtime when it worked with ‘limit 10000’?”


P: “Here’s a chart showing it bang-on at almost that exact same runtime for weeks, until it started timing out every single time on Friday night.”

J: “Okay, let us know when you’ve fixed that. Just for fun, try changing the query to just return the count of rows instead of the ~36 MB of data. I, um, have a hunch.”


U: “9.6 seconds.”

J: “So, you can successfully submit obnoxious queries through Partner’s interface, as long as they don’t return any significant quantity of data. Hmmm, what does that sound like?”


(long meeting full of fingerpointing with no indication of how it started failing 100% of the time like throwing a switch)

J: “I have finally managed to reproduce the failure locally, which means I’m willing to try a small config-file change to work around the problem. Reminder: we still have no hint as to the actual cause.”

[you are here]

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