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For now, our skeleton code bypassed part of the original datasource API for aggressive pushdown calculations into tikv's coprocessor and we don't uses stable api of Filters and prunes traits since they seems too narrow for coprocessor.

For calculation pushdown, what we do is direct matching subplan for pattern like Aggregates->Filter/Projection->TiRelation (for now, limit and sort not considered at least for May for simplicity)

We extract Filter/Projections and translate it to case AttributeReference -> ColumnReference for Coprocessor results case Filter and Projection Expression -> test if supported by tikv and translate to proto expressions, for the remaining part generate a supplementary plan on the top of TiRelation case Aggregate -> split into partial and final aggregate while partial part is translated into proto request, and final one is prepend on the top of Filter/Projection (if any, otherwise TiRelation directly); for any aggregates that cannot pushed, we need to fall all the way back to no aggregate-pushdown logic