[sigstoat]

> Adding an index now increases the insert operation cost/time

that insert is happening after you've checked the table to see if the record is present. so two operations whose times we care about are "select and accept email" and "select, tell the sender to come back in 20 minutes, and then insert".

the insert time effectively doesn't matter, unless you've decided to abandon discussion of the original table entirely without mentioning it.