Operator fusion: aggregates

[cybertyche]

At the moment, operator fusion of select, select many, where, and alter duration by constant operators is in place for ingress. Those operations can effectively become part of the ingress operation, localizing some of the computation.

We would like to do the same for aggregate operations, allowing that subset of operations to fuse into the output of an aggregate operator. The result will:

• Take a row as it is being added to an output batch and intercept it
• Apply the fused operations to that row
• Add the result to the output batch