Cost, in the context of the Visual Explain tool, is the estimated total resource usage necessary to execute the access plan for a statement (or the elements of a statement). Cost is derived from a combination of CPU cost (in number of instructions) and I/O (in numbers of seeks and page transfers).
The unit of cost is the timeron. A timeron does not directly equate to any actual elapsed time, but gives a rough relative estimate of the resources (cost) required by the database manager to execute two plans for the same query.
The cost shown in each operator node of an access plan graph is the cumulative cost, from the start of access plan execution up to and including the execution of that particular operator. It does not reflect factors such as the workload on the system or the cost of returning rows of data to the user.
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