Postgres string data type storage allocation

We are running Postgres 8.3.3 and the documentation (http://www.postgresql.org/docs/8.3/static/datatype-character.html) leaves me with a few questions about how storage is allocated.

F开发者_JS百科or the purposes of this discussion I'd like to stick to assuming a character = 1 byte.

I understand that character(n) blank pads so you always have n bytes stored.

If your char(n) > 126 characters long, you have an overhead of 4 bytes (presumably for storing the length), otherwise an overhead of 1 byte.

I'm interested in what happens with varchar(n).

If I use varchar(255) and store a 4 character string in it, do I have 4 bytes for the string & 4 bytes overhead, or do I have just one byte overhead until I hit the 126 character limit?

I've googled and can't find this documented anywhere.

Any thoughts?

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It's the length of the data you store that controls the overhead, not the maximum length of the column. So you have the one byte overhead until you hit 126.