Re: Storage requirements of NCHAR columns

On 10 Jul 2006 09:59:49 -0700, "Ross" <rossfreemantle_at_yahoo.co.uk> wrote:

>
>"When you use NCHAR and NVARCHAR2 datatypes for storing multilingual
>data, the
>column size specified for a column is defined in number of characters.
>(The number of
>characters means the number of Unicode code units.)"
>
>This sould seem to suggest that the a NCHAR(30) column would actually
>require 30 bytes of storage (as UTF8 has a single-byte code unit).
>However, an example in Chapter 7 explicitly states that 90 bytes are
>required. I don't think the NLS_LENGTH_SEMANTICS parameter affects
>NCHAR, so which is correct?
>

UTF8 is not a single-byte charset. It is a varying-width charset.

If you defines a NCHAR(30) column, it will contain 30 characters (unless NULL). Always. Period.

How many *bytes* of storage does this column require ?

That depends on which kind of characters you store in it. If you only store ASCII-characters, each of them will require one byte, because UTF8 is a superset of ASCII, ASCII being in the range 0-255.

If you store characters beyond ASCII, between 2 and 4 bytes of storage is needed, depending on what position these characters have in the UNICODE range, which is 0 - (2^32-1).

So a minimum of 30 bytes is required. And a maximum of 30*4 = 120 bytes, that if all your 30 characters are in the upper 2^8 end of the UNICODE range.

Conclusion : Between 30 and 120 bytes are required for a NCHAR(30) column with UTF8 as NCHARSET.

• Kenneth Koenraadt