The performance of a query against a table in the Windows Azure Table Service is critically dependent on whether or not the PartitionKey and RowKey are provided. The only index on a table is the combination of PartitionKey and RowKey. The most performant query, and the only one guaranteed not to result in a continuation token, is one specifying both PartitionKey and RowKey. A query specifying the PartitionKey but not the RowKey results in a scan of all the entities with that PartitionKey. A query specifying the RowKey but not the PartitionKey is processed sequentially by each partition server. A query specifying neither PartitionKey nor RowKey results in a scan of the entire table regardless of any filters in the query.
The Table service provides lower scalability targets for simultaneous queries within a single partition than it does for simultaneous queries against different partitions. Consequently, it can be more efficient to break a multipartition query...