What are the optimum varchar sizes for MySQL?

Question

How does MySQL store a varchar field? Can I assume that the following pattern represents sensible storage sizes :

1,2,4,8,16,32,64,128,255 (max)

A clarification via example. Lets say I have a varchar field of 20 characters. Does MySQL when creating this field, basically reserve space for 32 bytes(not sure if they are bytes or not) but only allow 20 to be entered?

I guess I am worried about optimising disk space for a massive table.

Answer 1

To answer the question, on disk MySql uses 1 + the size that is used in the field to store the data (so if the column was declared varchar(45), and the field was "FooBar" it would use 7 bytes on disk, unless of course you where using a multibyte character set, where it would be using 14 bytes). So, however you declare your columns, it wont make a difference on the storage end (you stated you are worried about disk optimization for a massive table). However, it does make a difference in queries, as VARCHAR's are converted to CHAR's when MySql makes a temporary table (SORT, ORDER, etc) and the more records you can fit into a single page, the less memory and faster your table scans will be.