Why not specify every VARCHAR as VARCHAR (65535)?

Question

Since the storage requirements for a Varchar field are based on the actual length of the string entered, what would be the downside of specifying every Varchar field as the max possible: Varchar (65535)? Well, aside from 1 extra byte for max fields > 255 characters?

[Storage Reqts for strings of length L: L + 1 bytes if column values require 0 – 255 bytes, L + 2 bytes if values may require more than 255 bytes]

Thank you!

Answer 1

From the documents - Table Column-Count and Row-Size Limits:

Every table (regardless of storage engine) has a maximum row size of 65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.

The maximum row size constrains the number (and possibly size) of columns because the total length of all columns cannot exceed this size. For example, utf8 characters require up to three bytes per character, so for a CHAR(255) CHARACTER SET utf8 column, the server must allocate 255 × 3 = 765 bytes per value. Consequently, a table cannot contain more than 65,535 / 765 = 85 such columns.

Storage for variable-length columns includes length bytes, which are assessed against the row size. For example, a VARCHAR(255) CHARACTER SET utf8 column takes two bytes to store the length of the value, so each value can take up to 767 bytes.

So, defining a single VARCHAR(65535) column, effectively limits you to a single column on the row (assuming you have filled it up).

All this apart from the fact that such a large size is completely wrong for some types of data - if you have a phone number column which may contain local and international numbers, you may choose to use a VARCHAR field to do so, but setting it to anything over 20 may well meaningless (I am being generous).

See this answer from Bill Karwin which also indicates possible performance penalties if temporary tables get generated with unnecessarily long VARCHAR fields (to do with conversion of such fields to CHAR and back again - see the post for details).

Answer 2

I think varchar column lengths are not only about storage. They're about data semantics as well.

I.E. specifying a name column as varchar(100) means that names stored on your system must not be longer than 100 characters.

On the storage side of things, they should be the same. Although, row size estimations would be more accurate with a specific length on varchar columns that without them (without needing a statistics gathering system keeping data distributions on varchar sizes).

Answer 3

One possible reason would be to improve compatibility with other applications. For instance, if you had an app that used a "product_no" field that was 100 chars long, and you wanted to interface with an app that used a similar field like "model_no" that was 40 chars long, it would be a pain. Any product_nos in your app that were longer than 40 chars would get truncated and you'd have to figure out some way to translate them between the applications.