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Is VARCHAR(100) any better than VARCHAR(500) from a performance point of view? What about disk usage?
Talking about PostgreSQL today, not some database some time in history.
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The answer is you don't need to, it's optional. It's there if you want to ensure that strings do not exceed a certain length.
String values that vary significantly in length and are no longer than 8,000 bytes should be stored in a VARCHAR column. If you have huge strings (over 8,000 bytes), then VARCHAR(MAX) should be used. In order to store VARCHAR columns, the length information along with the data is stored.
There is no performance impact whether you use the full length VARCHAR declaration VARCHAR(16777216) or use a smaller precision VARCHAR datatype column.
They are identical.
From the PostgreSQL documentation:
http://www.postgresql.org/docs/8.3/static/datatype-character.html
Tip: There are no performance differences between these three types, apart from increased storage size when using the blank-padded type, and a few extra cycles to check the length when storing into a length-constrained column. While character(n) has performance advantages in some other database systems, it has no such advantages in PostgreSQL. In most situations text or character varying should be used instead.
Here they are talking about the differences between char(n), varchar(n) and text (= varchar(1G)). The official story is that there is no difference between varchar(100) and text (very large varchar).
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There is no difference between varchar(m) and varchar(n)..
http://archives.postgresql.org/pgsql-admin/2008-07/msg00073.php
There is a difference between varchar(n) and text though, varchar(n) has a built in constraint which must be checked and is actually a little slower.
http://archives.postgresql.org/pgsql-general/2009-04/msg00945.php
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