Depends on you search pattern.
If you never care about scans ordering i.e. you only do point gets to see whether you've already seen an email address, do the hash part.
I'd perfer #1 over #2, because it would let you do efficient key prefix block encoding (FAST_DIFF).
From: Nurettin Şimşek <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Sent: Wednesday, February 13, 2013 12:35 AM
Subject: RowKey design with hashing
In our project mail adresses are row key. Which rowkey design we should
1) com.yahoo@xxxx (Reversed)
2) [EMAIL PROTECTED]
3) md5 hash([EMAIL PROTECTED])
4) Any other solution.
M. Nurettin ŞİMŞEK