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copied!<p>Partly, it depends on your DBMS. Some, like MySQL 5, care about the length of a VARCHAR(n) column as opposed to an unlimited-length TEXT column; others, like PostgreSQL, consider TEXT and VARCHAR(n) as internally identical, with the exception that a length is checked on VARCHAR(n) columns. Writing something like VARCHAR(65536) in PostgreSQL is silly; if you want an unlimited-length column, choose TEXT and be done with it.</p> <p>Of course, sometimes trying to store too long of a value will break your layout, or allow someone to abuse the system by choosing a long name with no spaces (for example). Usually what I do for username fields like that is just choose an upper length such that anyone who wants a longer username is trying to cause trouble; 64 characters is a nice round value and seems to work well. For real names and addresses (which aren't frequently displayed to users like a username is), you'll want to go with something longer. You want a value large enough that it can accept any valid input, but not so large that someone could stuff a gigabyte-long string in the field just to attack your system. 1024 characters is pretty reasonable: 1k is an small enough amount of text to easily work with, a round number, and larger than any sane address line or name.</p> <p><strike>Email addresses can, per the associated RFC whose number I am too lazy to look up right now, be no longer than 320 characters. So there's your email field length.</strike> Turns out that SMTP limits the length of fields to 256 characters; since email addresses must be bracketed, the longest valid email address is actually 254 characters. (<a href="http://www.eph.co.uk/resources/email-address-length-faq/" rel="noreferrer">This page</a> goes into more detail.) So <em>there's</em> your email field length.</p> <p>Passwords should <strong><em>NEVER</em></strong> be stored in plaintext, so your password field should be a byte array or BLOB type exactly long enough to store the output of the hash function you are using (or largest element of the cryptographic group in use, for more advanced schemes like SRP-6a).</p>

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