g., ID)
Rate of Change How fast the attribute changes (units/s)
Range Dynamic range of attribute values (pair of units)
Table 5 summarizes the metadata associated with each attribute, along
with a brief description. Attribute metadata is used primarily in two contexts:
(1) information about the cost and the time to fetch an attribute, and range
of an attribute which is used in query optimization, (2) information about the
semantic properties of attributes which is used in query dissemination and
result processing. Notice that the power consumption and time to sample can
di?®er across sensors by several orders of magnitude.
The catalog also contains metadata about TinyDB??™s extensible aggregate
system. The catalog includes names of aggregates and pointers to their code.
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Chapter 12 Data Management in Sensor Networks
Each aggregate consists of three functions, that initialize, merge, and update
the final value of partial aggregate records as they flow through the system.
The one that defines the extensible aggregate functions must provide information
about functional properties. In TinyDB, two properties must be satisfied:
that the aggregate is monotonic and whether it is exemplary or summary.
COUNT is a monotonic aggregate as its value can only become larger as
more values are aggregated. MIN is an exemplary aggregate, as it returns a
single value from the set of aggregate values, while AVERAGE is a summary
aggregate because it computes some result over the entire set of values.
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