A little while back I wrote about Why Indexes Reduce Locks for Update and Delete Queries.
I got a great question on the post from Zac:
What’s not super clear is why it takes out a lock on the whole table, is this because it does a lock escalation as a result of the Full Scan? Will this always happen, or is there a threshold of record update counts where this will occur?
This was tough to answer in just a comment, so I promised a full post on the topic.