In a dynamic switch search, the database first algorithm is not chosen by the IBM® Content Search Services server when the expected number of hits are more than the threshold value.
For example, with a single collection and a threshold of 10,000, if there are 20,000 or more CBR hits, dynamic switching tends to choose database first. But if there are only 15,000 CBR hits, there might be some searches that remain as CBR first searches because of the possible inaccuracy of the estimate.
If there is more than one index, the estimation algorithm assumes that the CBR hits occur uniformly across indexes, such as a search on a common word that can occur in many documents. Because the estimation algorithm searches the largest indexes first, if there are few hits in the initial indexes or any large indexes, the server estimation process might time out (the default is 30 seconds) or be terminated by the Content Platform Engine server. To ensure a quick dynamic count estimation time, counting will not continue past 12 collections if the CBR hits are not occurring uniformly and have a sufficient likelihood of being found across all collections.