1. A query plan execution system comprising: a memory; one or more processors; a plan guide metadata store that stores one or more plan guides that are each associated with one or more queries, wherein each plan guide comprises one or more hints that are supplied by a user independently of the creation of and without modifying the associated one or more queries, wherein each plan guide is one of multiple types including object, SQL, and template types; and an execution environment that, upon receiving a query to be executed, accesses the plan guide metadata store to determine whether the query is associated with one or more of the plan guides, and upon locating a matching plan guide, modifies at least one query statement of the query with the one or more hints of the matching plan guide such that the one or more hints are used to guide an optimization process that generates a query plan for the query wherein: when the query is part of a stored procedure, the matching plan guide is of type object, when the query is part of a batch of query statements, the matching plan guide is of type SQL, and when the matching plan guide is of type template, the matching plan guide directs the execution environment to force parameterization of the query.