explain actually runs the query. From http://www.mongodb.org/display/DOCS/Explain http://www.mongodb.org/display/DOCS/Explain : Note that explain runs the

explain actually runs the query. From http://www.mongodb.org/display/DOCS/Explain http://www.mongodb.org/display/DOCS/Explain : Note that explain runs the actual query to determine the result. If the query is slow, the explain will be slow too.