1. A computer-implemented method comprising: under control of one or more computer systems configured with executable instructions, receiving, via a network, a threshold parameter indicating a predetermined threshold similarity of subject matter; receiving, via the network, at least one input document associated with an item available for purchase, the at least one input document to be indexed into a search index file, wherein the at least one input document includes at least two fields, the at least two fields having content, and the predetermined threshold similarity of subject matter exists at least when a percentage of fields in a structure of the at least one input document correspond to fields in a structure of at least one existing document; comparing a subject matter of the at least one input document to a subject matter of the at least one existing document, wherein the at least one existing document relates to another item available for purchase and is already indexed into the search index file; determining, based at least on the comparison, that the at least one input document and the at least one existing document meet the predetermined threshold similarity of subject matter; responsive to determining that the at least one input document and the at least one existing document meet the predetermined threshold similarity of subject matter, comparing words in the at least one input document with words in the at least one existing document; designating any dissimilar words or phrases between the at least one input document and the at least one existing document as candidate synonyms; merging the candidate synonyms into the search index file, wherein the merging associates the dissimilar words or phrases as synonyms in the search index file; and indexing the at least one input document into the search index file.