The technology described herein provides for identifying matching documents for a search query using a bit vector search index. When a search query is received, a term is identified from the search index, and a number of bit vectors corresponding to the term are identified. Each bit vector comprises an array of bits in which at least one bit in each bit vector indicates that a corresponding document includes the term. Each bit vector also includes other bits indicating other documents include other terms. A determination is made that an unacceptable number of possible matching documents is likely to be returned. In response to the determination, a strengthening row bit vector is selected to reduce the number of possible matching documents. The identified bit vectors and the selected strengthening row are intersected to identify matching documents that contain the term.