[freetds] db-lib: support for new MS SQL 2008 data types - part 3

Frediano Ziglio freddy77 at gmail.com
Fri Apr 18 04:26:03 EDT 2014

2014-04-17 6:26 GMT+01:00 LacaK <lacak at zoznam.sk>:
> Frediano Ziglio  wrote / napísal(a):
>> 2014-04-16 12:44 GMT+01:00 LacaK <lacak at zoznam.sk>:
>>> Hi again,
>>> Now I am personally happy with patched db-lib in regards of support new
>>> DATE
>>> - TIME data types.
>> Great! Yesterday I discovered a problem with BCP adding some tests for
>> these new types, still to fix.
> May be, I have never used bcp_* functions ;-)

I'll fix it. It's a quite different code path compared to the one you
are working on. But is still related to same data types.

>>> But there are still missing some parts, which may be interesting to
>>> somebody
>>> else.
>>> I meant dbbind() family of API with corresponding *BIND constants and
>>> binary
>>> structure used to store this types.
>>> How to handle binding of new date, time types ?
>>> - introduce new DBDATETIMEALLBIND (or DBDATETIME2BIND) constant in
>>> sybdb.h ?
>> could work
>>> - introduce new DBDATETIMEALL struct (==TDS_DATETIMEALL struct) in
>>> sybdb.h
>>> ?
>> Mmm... well... could be or not. Microsoft for ODBC defined quite
>> different structures (one more similar to DBDATEREC). TDS_DATETIMEALL
>> is neither from TDS protocol neither intended to be presented to
>> clients. It's a mix of TDS protocols, numeric, old dates (values are
>> the same as dtdays).
>>  On the other end I could understand that
>> providing dbdata different from libTDS is far from easy.
> Exactlly!
> It is main reason why I am still speaking about TDS_DATETIMEALL ;-)
> And as I already wrote because of similarity of:
> SQL Server       libTDS                          DB-Lib
> ------------------------------------------------------------
> datetime        -> TDS_DATETIME   == DBDATETIME
> smalldatetime ->TDS_DATETIME4 == DBDATETIME4
> I will be happy also with SQL_TIMESTAMP_STRUCT (or other struct), to be
> public structure for these new date time data types, but IMO then this
> struct must be used also internaly by libTDS to store values in record
> buffer. Because if libTDS will continue use TDS_DATETIMEALL then it will
> significantly complicate things on db-lib level. (as there will be required
> conversion in many places)

Well.. TDS_DATETIME4 and TDS_DATETIME have same representation of wire
bytes (unless bit endian is different) and are documented in dblib.
TDS_DATETIMEALL is neither wire neither documented (so no ABI).
Unfortunately dbdata wants a binary representation of each data.
ctlib... I don't remember. ODBC either wants a bind or data get read
into user provided buffers (SQLGetData). Actually ODBC have separate
types for each MS type. The reason I added this libTDS type is that is
easier during the conversion to have a single type to work with.
Another reason is while wire for all date types are quite easy to put
directly into a structure these new types are quite different. The
size of date is 3 bytes so computers cannot handle directly (you need
to stick the 3 bytes into a single 32 bit integer) while seconds and
fraction size are from 3 to 5 bytes (same problem). This is the reason
for the two time and date fields. Obviously to store 5 bytes we need
at least a 8 byte integer. Somebody could say that an 8 bytes integer
is enough (3+5 = 8) and it's true but all datetime structure keeps
date and time separate and mostly of the time this would lead to just
some extra multiplication/division. Another thing about date. The zero
from the wire represent a date like 1-1-0... now, gregorian calendar
(the one we use) was introduced in 1592 so before they have different
calendar (month days and months order changed). So this zero is quite
artificial. This is why I preferred to set zero for this structure to
1-1-1900. About seconds wire send the number with precision so
00:00:01 is 1 for TIME(0) and is 100 for TIME(2). Actually the
structure always set this number as precision was 7. About bit fields.
These mainly are reduntant as they came directly from the type. They
are not on the data wire (precision is a field in the metadata), this
is similar to scale/precision for numeric data (which are in metadata
while we copy in libTDS data).

Well... all these looks quite paranoid but external ABI needs to stay
so is better to decide what to stick into the dbdata structure!
date: perhaps would be better to just store the number from wire
(converted to 32 bit) without bias;
time: perhaps would be better to just store the number from wire
time_spec: use 3 bit instead of 4 ? We just need a range from 0 to 7.
Another idea could be to use a single byte instead and separate all
other flags. As compiler usually reserve bits from the bottom and as
this bitfield is the first is much easier for the cpu to extract this
number. Personally I would keep the bitfield reducing to 3 bits.
has_time, has_date and has_offset: they are fine. The only change I
would insert a TDS_USMALLINT _res:10 before. In such was all the
single bits will occupy the top position leaving space for extensions.
Order of the fields are optimized to reduce structure size.

Do you think these changes are reasonable? Once decided we could
define a new DBDATETIMEALL and check for future ABI changes.

>>> - add support to copy_data_to_host_var() function in dblib.c ?
>> Ehmm... what do you mean?
> In db-lib is function copy_data_to_host_var() which is called when next
> record is fetched to store fetched column values to local program variables
> for bind columns.
> If you look at this function there are performed type conversions very
> simlar to dbconvert().
> So we must add there also cases for SYBMSDATE...SYBMSDATETIMEOFFSET (like we
> did for dbconvert())

Looks like extending dbconvert. Looks like a lot of duplicated code.
The base key is that dbconvert should convert between dblib type to
dblib type while copy_data_to_host_var should be conversion from
libTDS to dblib. The fact that dbdata returns mainly libTDS make the
functions very similar. Well... we could on the other way instead of
returning TDS_DATETIMEALL return different part of the structure for
different type, so for MSTIME we return the time, for MSDATE return
just date, MSDATETIME return first two fields and for MSDATETIMEOFFSET
return first 3 fields. There would still be the problem of getting
precision for the type.

>>> If you give me direction I can prepare patch ...
>>> (if it is good idea add support for BINDing ?)
>> Not good, excellent! And much easier as you don't have to store a
>> buffer but you can provide what do you want to the client.
> Yes
> So only think we must agree on is : what "struct" we will use for new date
> time data types ?
> If it will be TDS_DATETIMEALL then it makes things easy and compact.
> If it must be something else then my proposal is use same struct in libTDS
> instead of TDS_DATETIMEALL.

That is change TDS_DATETIMEALL or use as is !

> Thanks
> -Laco.


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