r/DBA

We had a query that ran fine for about a year. No code change, no new data, nothing deployed. One morning it was slow and the app team was on us.

The execution plan had changed. It picked a different join and fell over. Once I saw that, I pinned the plan and it was fine.

Working out that the plan had changed at all took most of the morning. Nothing flags it on its own, so I was comparing it by hand to an older plan.

how do you catch a plan change like this before users start complaining?