On Thu, Nov 03, 2005 at 03:09:26PM +0100, Martijn van Oosterhout wrote:
> On Thu, Nov 03, 2005 at 01:49:46PM +0000, Simon Riggs wrote:
> > In other databases, CHAR(12) and NUMERIC(12) are fixed length datatypes.
> > In PostgreSQL, they are dynamically varying datatypes.
> Please explain how a CHAR(12) can store 12 UTF-8 characters when each
> character may be 1 to 4 bytes, unless the CHAR itself is variable
> length...
> ...
> Nope, the verlena header stores the actual length on disk. If you store
> "hello" in a char(12) field it takes only 9 bytes (4 for the header, 5
> for the data), which is less than 12.
> ...
> Having a different header for things shorter than 255 bytes has been
> discussed before, that's another argument though.
It's unfortunate that the length is encoded multiple times. In UTF-8,
for instance, each character has its length encoded in the most
significant bits. Complicated to extract, however, the data is encoded
twice. 1 in the header, and 1 in the combination between the column
attribute, and the per character lengths.
For "other databases", the column could be encoded as 2 byte characters
or 4 byte characters, allowing it to be fixed. I find myself doubting
that ASCII characters could be encoded more efficiently in such formats,
than the inclusion of a length header and per character length encoding,
but for multibyte characters, the race is probably even. :-)
I dunno... no opinion on the matter here, but I did want to point out
that the field can be fixed length without a header. Those proposing such
a change, however, should accept that this may result in an overall
expense.
Cheers,
mark
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