Ocultos Exitus–JDBC Driver Unicode Settings and SQL Server Performance

While troubleshooting JDBC client apps that connect to SQL SErver I ran into this issue few times, latest very recently.

As you may already know well, SQL Server differentiates its data types that support Unicode from the ones that just support ASCII. For example, the character data types that support Unicode are nchar, nvarchar, longnvarchar where as their ASCII counter parts are char, varchar and longvarchar respectively. By default, all Microsoft’s JDBC drivers send the strings in Unicode format to the SQL Server, irrespective of whether the datatype of the corresponding column defined in the SQL Server supports Unicode or not.

In the case where the data types of the columns support Unicode, everything is smooth. But, in cases where the data types of the columns do not support Unicode, serious performance issues arise especially during data fetches. SQL Server tries to convert non-unicode datatypes implicitly in the table to unicode datatypes before doing the comparison. Moreover, if an index exists on the non-unicode column, it will be ignored. This would ultimately lead to a whole table scan during data fetch, thereby slowing down the search queries drastically.

This can be corrected by resetting one of the default parameter in the Java driver. The parameter name and value to be set might vary from driver to driver, depending on the vendor.

Vendor Parameter
JSQLConnect asciiStringParameters
JTDS sendStringParametersAsUnicode
DataDirectConnect sendStringParametersAsUnicode
Microsoft JDBC sendStringParametersAsUnicode
WebLogic Type 4 JDBC sendStringParametersAsUnicode

Reading – https://msdn.microsoft.com/en-us/library/ms378857(v=sql.110).aspx you will see- “ For optimal performance with CHAR, VARCHAR or LONGVARCHAR type of non-Unicode parameters, set the sendStringParametersAsUnicode connection string property to “false” and use the non-national character methods.” , issue was also reported via http://www.sqlconsulting.com/jdbc.shtml, http://www.jochenhebbrecht.be/site/2014-05-01/java/fixing-slow-queries-running-sql-server-using-jpa-hibernate-and-jtds and http://www.codeproject.com/Articles/281364/Solving-Performance-issues-in-data-migration-to-SQ

Hope this helps.

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