Re: How to find greatest record before known values fast

On 10/03/2014 01:28 PM, Andrus wrote:
> Hi!
>
> Thank you for explanations.
>
>> the char type pads out the fields on disk.
>
> It looks like you wrote that char takes more disk space.
>
> from
>
> http://www.pgcon.org/2013/schedule/attachments/269_tour-of-postgresql-data-types.pdf
>
>
> page 28:
>
> Unlike many
> databases, char(n) is NOT stored as afixed-sizedfield
> in Postgres. It is treated exactly the sameas
> varchar(n)except for being padded
>
> So char type does not take more space than varchar.

Which directly contradicts the information on page 27:

Character Types (or Strings)
Name
Description

varchar(n)
variable-length with limit

char(n)
fixed-length, blank padded

text
variable unlimited length

and the docs:

http://www.postgresql.org/docs/9.3/interactive/datatype-character.html

Values of type character are physically padded with spaces to the
specified width n, and are stored and displayed that way. However, the
padding spaces are treated as semantically insignificant. Trailing
spaces are disregarded when comparing two values of type character, and
they will be removed when converting a character value to one of the
other string types. Note that trailing spaces are semantically
significant in character varying and text values, and when using pattern
matching, e.g. LIKE, regular expressions.

Tip: There is no performance difference among these three types, apart
from increased storage space when using the blank-padded type, and a few
extra CPU cycles to check the length when storing into a
length-constrained column. While character(n) has performance advantages
in some other database systems, there is no such advantage in
PostgreSQL; in fact character(n) is usually the slowest of the three
because of its additional storage costs. In most situations text or
character varying should be used instead.

>
> Andrus.
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com