A method dynamically selects query execution operators for a database engine. The database engine receives a query and parses the query to form a query execution tree. The engine creates a first executable plan that includes in-memory operators, which execute within the volatile memory. While executing a first in-memory operator, the engine detects insufficient memory to complete the execution and aborts the execution. The engine then recompiles the query execution tree to form a second executable plan, which includes spooling operators. Each spooling operator executes within a fixed volatile memory budget. The engine executes the second executable plan, including the plurality of spooling operators, to identify a set of results from the database that is responsive to the query, and returns the results.