[Kde-pim] Akonadi db schema change needed to store search folder queryStrings

[Will Stephenson]

When using search folders in KMail (4.8) I see a lot of failed queries from 
Virtuoso on the command line.  The failed query is attempted repeatedly, 
causing extra cpu load.

The reason the query fails is that the search folder stores the indexing 
database query as a string in the QString queryString attribute on Collection, 
but in our MySQL schema, all QString attributes are stored as VARBINARY(255), 
which is too short for the sparql query used by Nepomuk. 

As a symptom, the first mail search works, I guess because the initial, 
complete query is passed to Nepomuk without being serialized to the database 
and restored in truncated form, but subsequent searches fail because the first 
search is repeated using the broken query.

Changing the field length of CollectionTable.queryStrings fixes the problem, 
done by manually running "alter table collectiontable change queryString 
queryString varbinary(1024)" in akonadiconsole.

Now I've tried to fix the root problem in the database schema code in 
akonadi/server/src/storage, but if I increase the field size for QString for 
VARBINARY above 255 in dbinitializer_p.cpp, this requires an additional byte 
per field of type QString across the entire database [1] This would be a good 
few more bytes in the database overall </understatement>.

I also tried to add a new data type LongQString to the schema definition 
files, that is mapped to VARBINARY(1024), but this fails because the generated 
entities.* files use the literal type attribute of the column as the c++ data 
type; there is currently no scope to map a symbolic type to an actual type.  
There is a "sqltype" attribute defined in the entities schema but this is 
unused, presumably because this was too exact in the face of different 
databases' SQL dialects.

Any better suggestions how to make room for the whole query string?

Will

[1] http://dev.mysql.com/doc/refman/5.6/en/char.html and following section
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