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The interface "Aggregator" is descibed described in details in the later section of the document (UDA).
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E.g.:
select avg( col2) from /portfolio // portfolio Valid
select col1, avg ( col2) from /portfolio portfolio (col1 should not be present in projection attributes) // invalid
Query Types & implementation:
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This will require us to maintain a registry of the aggregate functions defined & will also have to ensure that it gets created on newly joining nodes. We piggy back on the infra through which the OQL indexes get created on the peers & I assume that we can also create on newly joining nodes
UDA Schema exchange:
The UDA schema/definitions are exchanged between cluster nodes via profile exchange mechanism.