8. A computer-readable storage medium storing a set of code modules which when executed by a processor of a computer system cause the processor to search for e-mails, the computer-readable storage medium comprising: code for receiving information indicative of one or more search terms; code for determining a query plan based on the one or more search terms that specifies an interleaving between querying a first database in a plurality of databases that contains metadata associated with the e-mails and querying a second database in the plurality of databases that contains a full text index of the e-mails, the interleaving defining how results of a first database query against one database in the first or second databases influence a second database query against the other database in the first or second database; code for performing a search in response to the query plan to determine information related to the one or more e-mails; and code for generating a set of results based on the information related to the one or more e-mails.
8. A computer-readable storage medium storing a set of code modules which when executed by a processor of a computer system cause the processor to search for e-mails, the computer-readable storage medium comprising: code for receiving information indicative of one or more search terms; code for determining a query plan based on the one or more search terms that specifies an interleaving between querying a first database in a plurality of databases that contains metadata associated with the e-mails and querying a second database in the plurality of databases that contains a full text index of the e-mails, the interleaving defining how results of a first database query against one database in the first or second databases influence a second database query against the other database in the first or second database; code for performing a search in response to the query plan to determine information related to the one or more e-mails; and code for generating a set of results based on the information related to the one or more e-mails. 9. The computer-readable storage medium of claim 8 wherein the code for determining the query plan based on the one or more search terms comprises: code for determining a cost associated with executing the first database query against the first database; code for determining a cost associated with executing the second database query against the second database; code for determining the interleaving between querying the first database and querying the second database based on the cost associated with executing the first database query and the cost associated with executing the second database query; and code for generating the query plan using a combination of the first database query and the second database query.