MySQL data types for common columns

divided linked to some info that explains the basic issue (there's performance differences), but it's not simple enough to say that one's always better than the other. (otherwise, there'd be no reason to have both.) Also, in MyISM, the 64k max size for VARCHAR isn't per field -- it's per record.

Basically, there's 4 ways to store strings in database records:

1. fixed length
2. C-style strings (marked with a NULL or similar character at the end of the string)
3. Pascal style strings (a few bytes to indicate length, then the string)
4. Pointers (store the string somewhere else)

MyISM uses something similar to #3 for VARCHAR, and a hybrid approach for TEXT where it stores the beginning of the string in the record, then rest of the string somewhere else. InnoDB is similar for VARCHAR, but stores the complete TEXT field outside of the record.

With 1&4, the stuff in the record is always the same length, so it's easier to skip over if you don't need the string, but need stuff after it. Both #2 and #3 aren't too bad for short strings ... #2 has to keep looking for the marker, while #3 can skip ahead ... as the strings get longer, #2 gets worse for this particular use case.

If you actually need to read the string, #4 is slower, as you have to read the record, then read the string which might be stored elsewhere on the disk, depending on just how that database handles it. #1 is always pretty straightforward, and again you run into similar issues where for #2 gets worse the longer the string is, while #3 is a little worse than #2 for very small strings, but better as it gets longer.

Then there's storage requirements ... #1 is always a fixed length, so it might have bloat if most strings aren't the max length. #2 has 1 extra byte; #3 typically has 2 extra bytes if max length = 255, 4 extra bytes if a 64k max. #4 has the pointer length, plus the rules for #3 typically.

For the specific implementations within MySQL 5.1, the docs for MyISM state:

• Support for a true VARCHAR type; a VARCHAR column starts with a length stored in one or two bytes.
• Tables with VARCHAR columns may have fixed or dynamic row length.
• The sum of the lengths of the VARCHAR and CHAR columns in a table may be up to 64KB.

While for InnoDB :

• The variable-length part of the record header contains a bit vector for indicating NULL columns. If the number of columns in the index that can be NULL is N, the bit vector occupies CEILING(N/8) bytes. (For example, if there are anywhere from 9 to 15 columns that can be NULL, the bit vector uses two bytes.) Columns that are NULL do not occupy space other than the bit in this vector. The variable-length part of the header also contains the lengths of variable-length columns. Each length takes one or two bytes, depending on the maximum length of the column. If all columns in the index are NOT NULL and have a fixed length, the record header has no variable-length part.
• For each non-NULL variable-length field, the record header contains the length of the column in one or two bytes. Two bytes will only be needed if part of the column is stored externally in overflow pages or the maximum length exceeds 255 bytes and the actual length exceeds 127 bytes. For an externally stored column, the two-byte length indicates the length of the internally stored part plus the 20-byte pointer to the externally stored part. The internal part is 768 bytes, so the length is 768+20. The 20-byte pointer stores the true length of the column.

...

as with so many other things when dealing with databases, if you're not sure what's best for your needs, try benchmarking it with similar data & usage, and see how they behave.

I don't know what the best way necessarily is to store it -- but there's at least a better option than using a varchar(39) (or varchar(40) if you needed it signed) ; instead use a decimal(39,0). From the mysql docs:

Fixed-Point (Exact-Value) Types

The DECIMAL and NUMERIC types store exact numeric data values. These types are used when it is important to preserve exact precision, for example with monetary data. In MySQL, NUMERIC is implemented as DECIMAL, so the following remarks about DECIMAL apply equally to NUMERIC.

MySQL 5.1 stores DECIMAL values in binary format. Before MySQL 5.0.3, they were stored as strings. See Section 11.18, “Precision Math”.

In a DECIMAL column declaration, the precision and scale can be (and usually is) specified; for example:

salary DECIMAL(5,2)

In this example, 5 is the precision and 2 is the scale. The precision represents the number of significant digits that are stored for values, and the scale represents the number of digits that can be stored following the decimal point.

Standard SQL requires that DECIMAL(5,2) be able to store any value with five digits and two decimals, so values that can be stored in the salary column range from -999.99 to 999.99.

In standard SQL, the syntax DECIMAL(M) is equivalent to DECIMAL(M,0). Similarly, the syntax DECIMAL is equivalent to DECIMAL(M,0), where the implementation is permitted to decide the value of M. MySQL supports both of these variant forms of DECIMAL syntax. The default value of M is 10.

If the scale is 0, DECIMAL values contain no decimal point or fractional part.

The maximum number of digits for DECIMAL is 65, but the actual range for a given DECIMAL column can be constrained by the precision or scale for a given column. When such a column is assigned a value with more digits following the decimal point than are permitted by the specified scale, the value is converted to that scale. (The precise behavior is operating system-specific, but generally the effect is truncation to the permissible number of digits.)

It's stored packed, so it'll take up less space than the varchar (18 bytes, if I'm doing my math right), and I'd hope you'd be able to do math on it directly, but I've never tried with that large of a number to see what happens.