Re: indexing on char vs varchar

I have updated the FAQ to mention that char()/varchar()/text/bytea have
similar performance characteristics.

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Josh Berkus wrote:
> Beth,
>
> > Sorry....I don't understand. The length is at the front of what?
>
> In some RDBMSs, the VARCHAR data type has a 2 or 4-byte indicator of the
> length of the stored string before the data itself, while CHAR does not
> require this information because it is fixed-length. This makes the CHAR
> datatype marginally smaller, and thus faster, than the VARCHAR data type on
> those databases. This difference goes back to much older databases and
> computers, where every byte of a row counted in terms of performance.
>
> In my experience, even though MS SQL Server still functions this way, the
> performance difference between CHAR and VARCHAR is not measurable unless you
> are getting close to the 8K data page limit that MSSQL imposes. YMMV.
>
> Postgres does not materially differentiate between CHAR, VARCHAR, and TEXT,
> except that CHAR is padded by spaces and VARCHAR often has a length limit.
> However, in terms of storage efficiency (and indexing efficiency), they are
> identical. In Postgres, the character count is included in all string data
> types.
>
> Thus, you should use the data type most appropriate to the data you are
> storing, ignoring performance issues. If the data is a fixed-length string
> (such as a required zip code) use CHAR; if it's variable but limited, use
> varchar; if it's a long description, use TEXT.
>
> --
> -Josh Berkus
> Aglio Database Solutions
> San Francisco
>
>

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