Okay, I'll admit it: SQL and big data things aren't at the top of my skillset.
No earlier than today, I resolved an issue we had in our customer databases: adding a missing index.
Seems easy at first but if the table is HUGE (yea, 8GB+ is big for me) with about 246MM rows (!!), it gets damn slow and the downtime doesn't last for an eye blink.
ALTER TABLE allows read operations while altering the table then blocks reads and writes ops when the alteration is done and needs to go live. This may vary according to the configuration