FreeTDS Development Group

[jklowden AT schemamania.org]

On Fri, Feb 05, 2010 at 04:38:31PM +0100, Frediano Ziglio wrote:
> 2010/2/5  <jklowden AT schemamania.org>:
> > On Fri, Feb 05, 2010 at 03:57:03PM +0100, Frediano Ziglio wrote:
> >> Using nchar(10) "foo" get stored as "foo       " ("foo" followed by 7
> >> spaces). Using utf-8 encoding a 5 character get stored in 10 nchar but
> >> if a character is not ascii get encoded in more than 10 characters.
> >> SQLDescribeCol return 10 characters and you provided 10 character
> >> buffer which is insufficient to store original data. Now... On
> >> SQLDescribeCol
> >>
> >>  ColumnSizePtr
> >>
> >>     [Output] Pointer to a buffer in which to return the size (in
> >> characters) of the column on the data source.
> >>
> >> so is fine to return 10 or we should return that maximum buffer space
> >> (40 in this case)    Or perhaps we should return 0 (undeterminated)  
> >
> > SQLDescribeCol is correct.  It returns the logical size, the size as 
> > reported by the server.  The column size is measured in characters, by 
> > definition; the server's storage requirements are not the client's 
> > concern.
> >
> > To determine the appropriate buffer size, bsqlodbc should use 
> > SQLGetDescRec and SQL_DESC_OCTET_LENGTH.


Hi Freddy, 

> correctly our imlpementation returns
> 10 and.... (rumble!) SQL_WCHAR... I forget this SMALL detail.... 

What should it return if the encoding is UTF-8?  SQL_WCHAR seems correct, 
unless it means UCS-2LE.  I think SQL_WCHAR means "Unicode".  

> I think that our driver
> is 100% correct... 

SQLDescribeCol correctly returns ColumnSizePtr = 10 for nchar(10), yes.  

> it detects truncation! 

Hurrah!  :-)

> At this point I would ask me
> if SQL_DESC_OCTET_LENGTH is correct... but perhaps we can't even use
> this value to compute character buffer... 

The ODBC specification does not contemplate UTF-8.  It assumes UCS-2.  So 
we're extending it, and we need to remember POLA.  

SQL_DESC_OCTET_LENGTH is the length in bytes of the buffer needed hold the 
data.  The driver knows how the buffer is encoded.  It should return -- as 
dbcollen() does -- the maximum size that could be required to hold any value 
that the column could hold.  For nchar(10) in UTF-8, that's 40.  

Now let me grumble for a moment, OK?  This is a perfect example of ODBC's 
needless complexity.  Who *cares* about the server's idea of the length?  The 
application needs to know how many bytes to allocate, calls SQLDescribeCol, 
and gets a useless answer.  So, it has to call SQLAllocHandle to get a 
SQLHDESC and call SQLGetDescField!  Wouldn't it be better to have a 
bytes-per-character function that takes a type as an input and returns a 
size? 

Mind, there's no consistency.  If SQLDescribeCol returns the logical length, 
in characters, of nchar, why doesn't it return 1 for TINYINT, SMALLINT, INT, 
and BIGINT?  After all, they're one logical length, and SQLDescribeCol 
doesn't describe *storage*, right?  Or perhaps SQLDescribeCol's job is only 
to report what the server says, and the *server* is inconsistent!  

OK, I'm done.  I hope to try SQL_DESC_OCTET_LENGTH this weekend.  

Regards, 

--jkl