1. A method for generating a query results preview, the method comprising: receiving a preliminary query while a query for a search is being constructed; determining, using a processor, a distribution of result items that would be retrieved if the search were executed using the preliminary query, including a first proportion of the result items in a new result category comprising new result items that have not been previously retrieved in response to a previous search by a user in a session and a second proportion of the result items in a previous result category comprising result items previously retrieved in response to a previous search in the session; generating the query results preview using the distribution of result items that would be retrieved if the search were executed using the preliminary query; and providing the query results preview while the query for the search is being constructed and prior to execution of the search using the query, wherein the query results preview comprises a visualization of the distribution of result items that would be retrieved if the search were executed using the preliminary query, including the first proportion of the result items in the new result category and the second proportion of the result items in the previous result category, the first proportion and the second proportion being separately displayed in the query results preview.