Oracle Performance terrible after changing Varchar2 fields to NVarchar2

Question

Hi All

Any help or suggestions would be greatly welcome on this. I've been developing a DotNet project on oracle (Ver 10.2) for the last couple of months and was using Varchar2 for my string data fields. This was fine and when navigating the project page refreshes were never more than a half second if even (it's quiet a data intensive project). The data is referenced from 2 different schemas, one a centralised store of data and one of which is my own. Now the centralised schema will be changing to be unicode compliant (but hasn't yet) so all Varchar2 fields will become NVarchar2, in preparation for this I changed all the fields in my schema to be NVarchar2 and since then performance has been horrible .. up to 30/40 second page refreshes.

Could this be because Varchar2 fields in the centralised schema will be joined against NVarchar2 fields in my schema on some stored procedures. I know NVarchar2 is twice the size of Varchar2 but that wouldn't explain the sudden massive change. As I said any tips for what to look for to improve would be great, if I haven't explained the scenario well enough do ask for more information.

Regards

Answer 1

Israfel,

It is difficult to say anything based on what you have provided. Did you manage to check if the estimated cardinalities and/or explain plan changed when you changed the datatype to NVARCHAR2? You may want to read the following blog post to see if you can find a lead http://joze-senegacnik.blogspot.com/2009/12/cbo-oddities-in-determing-selectivity.html

Answer 2

It is likely no longer able to use indexes that it previously could. As Narendra suggests check the explain plan to see what changed. It is possible that once the centeralized store is changed the indexes will again be usable. I suggest testing that path.

Answer 3

Firstly, do a

select * from v$nls_parameters where parameter like '%SET%';

Character sets can be complicated. You can have single-byte charactersets, fixed-size multibyte character set sand variable-sized multi-byte character sets. See the unicode descriptions here

Secondly, if you are joining a string in a single-byte characterset to a string in a two-byte characters set, you have a choice. You can do a binary/byte comparison (which generally won't match anything if you compare between a single-byte character set and a two-byte characterset). Or you can do a linguistic comparison, which will generally mean some CPU cost, as one value is converted into another, and often the failure to use an index.

Indexes are ordered, A,B,C etc. But a character like Ä may fall in different places depending on the Linguistic order. Say the index structure puts Ä between A and B. But then you do a linguistic comparison. The language of that comparison may put Ä after Z, in which case the index can't be used. (Remember your condition could be a BETWEEN rather than an = ).

In short, you'll need a lot of preparation, both in your schema and the central store, to enable efficient joins between different charactersets.