Your search field would probably need a valueChangeListener method which if residing in xBean then whatever xBean.aTable is returning, i.e. the IUData component, then can set the first row to be shown as zero, i.e. UIData.setFirst(0);
I'd be interested to know how many times per request the db query is being executed for your table data, given that you're also sorting on the columns, from what I've gone through, your question about reseting the row index would of been needed to prevent duplicate queries when sorting the dataTable ? Also, Is your xBean in the session or request scope ?