1. A computer-implemented method for generating a universal query result set, comprising: under control of one or more computer systems configured with executable instructions, submitting a received query to a plurality of search indexes, each search index associated with a relevance ranking function for ranking items according to an individual ranking scale, at least two of the individual ranking scales being different; receiving from the plurality of search indexes a plurality of search index result sets, each search index result set including one or more items related to the received query and ranked according to one of the individual ranking scales; determining a query index association for each of the plurality of search indexes; and including in the universal query result set a list of items obtained from different ones of the plurality of search index result sets, the list being ordered based at least in part on a probability of the items satisfying the query, the probability of the items satisfying the query determined at least in part by: computing an allocation score for each of the plurality of search indexes, the allocation score for each search index being computed based at least in part upon a relative number of search results in the corresponding search index result set; for each of the plurality of received search index result sets, computing a universal item score for at least one item identified in the search index result set, the universal item score for each identified item being computed at least in part by normalizing the individual ranking scale for each search index result set to a common ranking scale; and determining a probability of each item satisfying the query based at least on a combination of the allocation score for the search index associated with the item and the universal item score for the item.