This disclosure relates to increasing performance of database queries. A proxy server receives an input query string and a parameter value for first parameter name in the query string. The proxy server determines a second parameter name based on the parameter value and different to the first parameter name. The proxy server then determines an output query string based on the input query string. The output query string comprises a filter clause with a field name and a second field value, the second field value of the output query string being based on the second parameter name. The proxy server finally sends the output query string to a database management system to cause the database management system to execute a database query using an execution plan based on the second parameter name in the output query string.