Friday, March 06, 2009

Oracle OLE DB provider precision issues with SSIS 2005

We had this problem[3] before where SSIS would keep complaining that the Oracle client wasn’t giving it the correct precision information for numeric columns:

Error at Import Broken Stock On Hand Levels [DTS.Pipeline]: The "output column "WORKD_TONNES" (42)" has a precision that is not valid. The precision must be between 1 and 38.

We were using Oracle 9r2 client + OLE DB provider against an Oracle 8 database, and I just assumed that one of those wasn’t passing though metadata it should have, leaving SSIS to see the precision and scale as 0:


You can explicitly set the scale/precision in the advanced editor, but it keeps wanting to overwrite it every time you look at the query, which is a major pain in the arse. And explicitly TRUNCing or ROUNDing the column in the source query doesn’t seem to make any difference, even while you’d think that would make the precision explicit. So instead we used the Microsoft OLE DB Provider for Oracle (MSDAORA), and everything was happy.

Well nearly.

Actually it turns out that rather than MSDAORA ‘working’, all it actually does is default the precision and scale as 38,0 in scenarios where it can’t determine the precision/scale. And this happens rather a lot, since an Oracle Numeric doesn’t actually have to have a length or precision specified (though it should): it can be some kind of wierd variable precision float/numerical hybrid, which isn’t something that SSIS 2005 really caters for (nor other parts of Sql for that matter). Anyway, the point is that 38,0 isn’t the right answer either: whilst it’s fine for integer data anything after the decimal is lost, and that can take a while to pick up on.

So I’m not sure which is worse. On the one hand you can use the Oracle OLE DB driver, and all your precisions will be wrong until you fix them, but at least it’s in your face enough you know about it (though sometimes you can’t even OK the dialog, to get into the advanced editor and fix it, and/or it whinges about mis-matches with error output columns).

On the other hand the Microsoft driver makes a good stab at ‘what you probably want’, but as a result you lose precision on decimal data. And even if you set the precision up, it’ll overwrite it again (silently) when you touch the query, so it can go from working to not very inconspicously.

Probably the only safe way here is to use the Microsoft driver, and modify all your queries to cast all non-integer variable Numeric columns to strings[2]. That way the conversion into decimal data has to be explicitly defined, and can’t be lost quite as easily. It’s a bit more work of course, but at least the output is right.

Which brings me back to why I started looking at this again because there is no 64 bit version of MSDAORA [1] (I’m not even sure if MSDAORA is actively supported anymore), and we’ve had all manner of fun getting the 32 bit version running talking to either of the 32 and 64 bit versions of the Oracle client. We failed, but I may have just configured it wrong.

So instead we are in this ridiculous situation where we use MSDAORA at design time, and the Oracle OLE DB driver on the server at execution time, which creates all manner of warnings in the logs and also means some Sql (with inline comments) works on a developers machine but doesn’t work on the server.

It’s a mess.

[1] Nor Jet either, if you were crazy enough to load something from an Excel file. Who would do such a thing?
[2] Interestingly this is exactly how SSIS 2008 gets round this problem: it gives up, maps the variable-Numeric column as a string and leaves the conversion up to you
[3] Obviously the first problem we had was the whinging about the DefaultCodePage issue
[4] Managed to get all the footnotes in backwards for this post, which is pretty poor.

1 comment:

Anonymous said...

In the Oracle SQL cast your column with something like:

select cast(patient_id as numeric(10,0)) as patient_id
from patients

Obviously if you need a precision, then put that in too. Another problem you might encounter is when you have a union, in this case you can't cast the number in each select statement of the union, you have to put the union in as an inline select and put the cast in the outer query, e.g.:

select cast(patient_id as numeric(10,0)) as patient_id
(select patient_id from patients
where Title='Mr'
and date_of_birth >
select patient_id from patients
where Title='Miss')

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