is varchar(n) saving memory space in postgresql? [closed] - postgresql

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If I define a varchar(25) and my string is less than 25 characters (e.g. 12 chars), SQL shows the vector of char as a length of 12 and no trailing spaces are added (unlike character does).
My question is the following : behind the fact that SQL displays just the string as it was inserted in the field (or truncated if longer than the maximum length), how Postgresql is storing such a data type?
Is it padding with extra bytes as :
twelvecharxx............. (length : 25)
or just storing 12 bytes?
I assume this could be more complex internally. I just need to know if the maximum-length optional argument is a safety to disallow large string storage or just a performance question (regarding if all the subsequence stored strings are expected to be less or equal to 25 characters).

Answer 1

SQL defines two primary character types: character varying(n) and
character(n), where n is a positive integer. Both of these types can
store strings up to n characters in length. An attempt to store a
longer string into a column of these types will result in an error,
unless the excess characters are all spaces, in which case the string
will be truncated to the maximum length. (This somewhat bizarre
exception is required by the SQL standard.) If the string to be stored
is shorter than the declared length, values of type character will be
space-padded; values of type character varying will simply store the
shorter string.
and
The storage requirement for a short string (up to 126 bytes) is 1 byte
plus the actual string, which includes the space padding in the case
of character. Longer strings have 4 bytes overhead instead of 1.
finally
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.
from here

Answer 2

No padding is stored for varchar, according to the documentation. For character it is, however. There's also some overhead in storing the length of the string.