During my work with companies using MySQL, I have encountered many issues with regard to schema design, normalization and indexing. Of the most common errors are incorrect data types definition. Many times the database is designed by programmers or otherwise non-expert DBAs. Some companies do not have the time and cannot spare the effort of redesigning and refactoring their databases, and eventually face poor performance issues.
Here's a compilation of "the right and the wrong" data types.
INT(1)is not one byte long.
INT(10)is no bigger than
INT(2). The number in parenthesis is misleading, and only describes the text alignment of the number, when displayed in an interactive shell. All mentioned types are the same INT, have the same storage capacity, and the same range. If you want a one-byte
- An integer
PRIMARY KEYis preferable, especially if you're using the InnoDB storage engine. If possible, avoid using
PRIMARY KEY. In InnoDB, this will make the clustered index deeper, secondary indexes larger (sometimes much larger) and look ups slower.
- Do not use
VARCHARto represent timestamps. It may look like
'2008-11-14 07:59:13'is a textual field, but in fact it's just an integer counting the seconds elapsed from 1970-01-01. That's 4 bytes vs. 19 if you're using
ASCIIcharset, or more if you're using
- Do not use
VARCHARto represent IPv4 addresses. This one is quite common. The IP 192.168.100.255 can be represented with
VARCHAR(15), true, but could be better represented with a 4-byte int. That's what IPv4 is: four bytes. Use the
INET_NTOA()functions to translate between the INT value and textual value.
- This one should be obvious, but I've seen it in reality, where the schema was auto generated by some naive generator: do not represent numbers as text. Yes, I have seen integer columns represented by
VARCHAR. Don't ask how the performance was.
MD5()columns shouldn't be
CHAR(32)instead. It's always 32 bytes long, so no need for
VARCHAR's additional byte overhead. If your tables or database are
UTF8by default, make sure the MD5 column's charset is
ASCII, or it will consume 96 bytes instead of just 32. I also suggest the case-sensitive
ascii_bincollation, but that's a more minor issue.
PASSWORD()columns shouldn't be
CHAR. The length depends on whether you're using
old-passwordsvariable (for some strange reason, this variable always appears in the MySQL sample configuration files - though you really don't want it unless it's for backward compatibility with older MySQL versions). As in the MD5 note, use
- Better use
INTto count seconds, as MySQL has many supportive functions for this data type.
INTwhen possible. Do you expect to have 4000000000 customers? No? Then a "
id SMALLINT" may suffice as
CHARACTER SETs with care. More on this on future posts.