1. A computer-implemented method for managing a distributed database, comprising: receiving a query defining a first workflow, the first workflow corresponding to tasks for processing data from one or more data sources, the query having a corresponding first logical expression comprising a sequence of operators from an expanded relational algebra, at least one operator in the first logical expression being an instance of a reduce operator; modifying an ordering of the operators in the first logical expression to form a modified first logical expression, the modifying of the ordering of the operators in the first logical expression comprising a) moving one or more operators from a position prior to the instance of the reduce operator to a position after the instance of the reduce operator, b) moving one or more operators from a position after the instance of the reduce operator to a position prior to the instance of the reduce operator, or c) a combination thereof; constructing a modified first workflow corresponding to the modified first logical expression; and executing at least a portion of the modified first workflow, wherein the modified first workflow has a lower execution cost than an execution cost of the first workflow.