12. The system of claim 9 , wherein the selected partition of the user-term index comprises a plurality of database shards organized by time, and the indexing module is further configured to: select a record in a most recent database shard of the plurality of database shards, the record comprising the user identifier, the term identifier, and a list of post identifiers; and add the post identifier into the list of post identifiers of the selected record in the most recent database shard.
12. The system of claim 9 , wherein the selected partition of the user-term index comprises a plurality of database shards organized by time, and the indexing module is further configured to: select a record in a most recent database shard of the plurality of database shards, the record comprising the user identifier, the term identifier, and a list of post identifiers; and add the post identifier into the list of post identifiers of the selected record in the most recent database shard. 16. The system of claim 12 , wherein the number of database shards fluctuates over time.