FreeTDS Development Group

["James K. Lowden" <jklowden AT schemamania.org>]

On Wed, 8 Oct 2003, "ZIGLIO Frediano" <Frediano.Ziglio AT vodafone.com>
wrote:
> 
> Every TDSCOLINFO has a iconv_info field. This for compatibility with
> mssql2k (where every column can have a different type). 
> We don't use this field very much. I think we should set this 
> field for
> every column that require a conversion (even using TDS5!) and adjust
> column size even for single byte column (like VARCHAR, not only
> NVARCHAR). In this way we can just use this pointer in 
> tds_get_char_data (read.c). 

Yes.  That was my intention.  

One thing I've learned lately: there has to be a way to turn off
single-byte
conversions.  The client might want the data as-is.  

> This pointer should point to a structure in tds->iconv_info,
> however this array must be resized so the pointer could change. 

Yes, but resizing is avoidable.  See below.  

> solution should use a TDSICONVINFO ** for tds->iconv_info and allocate
> every TDSICONVINFO separately. 

That's inefficient.  For a given connection, there are only a few
conversion
pairs.  The client's charsets are invariant, and the server normally
encodes
character data in only one or two ways (that a single client cares about).
 

For a connection, I expect the TDSICONVINFO array to be 4-6 elements,
practically never more than 10.  If you allocate per-column, you'll
allocate
(and deallocate) thousands of TDSICONVINFO structures during the lifetime
of
a connection.  Instead of 5 or so.  

> At this point I suggest to replace
> iconv_info_count/iconv_info with a iconv_info_private (or 
> something like
> that) structure. So we separate TDSICONVINFO handling in iconv.c
> entirely (using iconv_info in column we avoid the need to access to
> tds->iconv_info as an array).

If TDSCOLINFO::TDSICONVINFO is set correctly, it's not necessary for
column
processing to have access to the socket.  We're really just talking about
allocation, not scope.  

The only time the array size changes is when a new conversion is required
by
a column, while processing a ROW_FMT packet.  It's not hard to modify the
array first (if necessary) and then set each column's TDSICONVINFO
pointer.


> Time ago I was thinking about changing TDSICONVINFO structure removing
> charset names and just storing our canonic index. In such way 
> we reduce memory usage and improve performance. 

I like having the string, because it makes debugging easier.  I agree that
comparing two ints is faster than strcmp(), but I really don't thinks it
matters.  Perhaps we can put this off until version 0.70 or something,
when
iconv is completely stable and well understood?  

> Another idea using directly TDSICONVINFO (always as a pointer, without
> pointer operations) is the possibility to define a private 
> TDSICONVINFO separating other possible iconv implementations.

I don't think I know what you mean here.  What "other possible iconv
implementations" are you thinking of?  

Regards, 

--jkl