I ran explain (F6) on a query yesterday. The estimated time was a few minutes and it took 6 minutes to run.
I pulled up the exact same query today and ran explain, but it produced an estimated time of 65,771 hours. However, I decided to run the query anyway and it still ran in 6 minutes.
1. Why would explain give totally different time estimates for the same query, and
2. Why would the latter estimate be so much higher than its actual runtime?
I used to look at this time as an estimate for how efficient my queries were and how long they would take to run, but it seems like the time estimate can't be trusted.