I have an indexed column that stores an MD5 hash. Thus, the column will always store a 32-character value. For whatever reason, this was created as a varchar rather than a char. Is it worth the trouble of migrating the database to convert it to a char? This is in MySQL 5.0 with InnoDB.
A similar question was asked before
Here is the excerpt of my answer
You must realize the tradeoffs of using CHAR vs VARCHAR
With CHAR fields, what you allocate is exactly what you get. For example, CHAR(15) allocates and stores 15 bytes, no matter how characters you place in the field. String manipulation is simple and straightforward since the size of the data field is totally predictable.
With VARCHAR fields, you get a completely different story. For example VARCHAR(15) actually allocates dynamically up to 16 bytes, up to 15 for data and, at least, 1 additional byte to store the the length of the data. If you have the string 'hello' to store that will take 6 bytes, not 5. String manipulation must always perform some form of length checking in all cases.
The tradeoff is more evident when you do two things: 1. Storing millions or billions of rows 2. Indexing columns that are either CHAR or VARCHAR
TRADEOFF #1 Obviously, VARCHAR holds the advantage since variable-length data would produce smaller rows and, thus, smaller physical files.
TRADEOFF #2 Since CHAR fields require less string manipulation because of fixed field widths, index lookups against CHAR field are on average 20% faster than that of VARCHAR fields. This is not any conjecture on my part. The book MySQL Database Design and Tuning performed something marvelous on a MyISAM table to prove this. The example in the book did something like the following:
This directive forces are VARCHARs to behave as CHARs. I did this at my previous job back in 2007 and took a 300GB table and sped up index lookups by 20%, without changing anything else. It worked as published. However, it did produce a table almost double in size, but that simply goes back to tradeoff #1.
You could analyze the data being stored to see what MySQL recommends for column definition. Just run the following against any table:
This will traverse the entire table and recommend column definitions for every column based on the data it contains, the minimum field values, maximum field values, and so forth. Sometimes, you just have to use common sense with planning CHAR vs VARCHAR. Here is a good example:
If you are storing IP addresses, the mask for such a column is at most 15 characters (xxx.xxx.xxx.xxx). I would jump right at CHAR(15) in a heartbeat because the lengths of IP addresses will not vary all that much and the added complexity of string manipulation controlled by an additional byte. You could still do a PROCEDURE ANALYSE() against such a column. It may even recommend VARCHAR. My money would still be on CHAR over VARCHAR in this instance.
CHAR vs VARCHAR issues can be resolved only through proper planning. With great power comes great responsibility (cliche but true)
When it comes to MD5 the computation of strlen internally should be eliminated when switching the entire row format. There would be no need to change the field definition.
If the MD5 key is the only VARCHAR present, I would go for it and convert the table row format to fixed. If there is a signifcant number of other VARCHAR fields present, they would benefit as well. In exchange, the table would expand to about twice its size. But queries should accelerate about 20% more without additional tuning.
Thanks to Ovais (see comments) for pointing out that
It is not worth changing in my opinion. If you look through the documentation here it should illustrate the difference between the two. In your usage scenario the one doesn't really offer any significant benefit over the other unless you are really concerned about the extra bit of overhead related to row size.
Also note the first comment on the documentation I link to above... "CHAR will only speed up your access if the whole record is fixed size. That is, if you use any variable size object, you might as well make all of them variable size. You gain no speed by using a CHAR in a table that also contains a VARCHAR"