check this out: https://news.ycombinator.com/item?id=20414721

"Also at that time, I remember asking the head DB guy about a specific metric, and he ran a live query against the database in front of me. It took a while to return, so he used the time to explain how, in an ordinary setup, the query would have locked all the tables and brought down the entire site, but he was using special SQL-fu to make it run transparently.

We got so engrossed in the details of this topic that half an hour passed before we noticed that everyone had stopped working and was running around in a frenzy. Someone finally ran over and asked him if he was doing a query, he hit Control-C, and Twitter came back up. "