Cfr: REG:117112417214967

When you use a query object and use the firstonly property on the first QueryBuildDataSource this is not translated to SQL as a SELECT TOP 1 only the kernel fetches the first record. There are workarounds using position based paging but in my opinion this is not the intended behaviour and can lead to poor performance.

(I have tested this on Ax 2012 R3 CU13 and Dynamics 365 For Finance and Operations PU11 Spring 2017)