A scheduler for a search engine crawler includes a history log containing document identifiers (e.g., URLs) corresponding to documents (e.g., web pages) on a network (e.g., Internet). The scheduler is configured to process each document identifier in a set of the document identifiers by determining a content change frequency of the document corresponding to the document identifier, determining a first score for the document identifier that is a function of the determined content change frequency of the corresponding document, comparing the first score against a threshold value, and scheduling the corresponding document for indexing based on the results of the comparison.