This PR extends #9209. It consists of 2 main points:
To enable parallelization of user-defined aggregates, reduction function was added to UDA definition. Reduction function is optional and it has to be scalar function that takes 2 arguments with type of UDA's state and returns UDA's state
All currently implemented native aggregates got their reducible counterpart, which return their state as final result, so it can be reduced with other result. Hence all native aggregates can now be distributed.
Local 3-node cluster made with current master. `node1` updated to this branch. Accessing node with `ccm <node-name> cqlsh`
I've tested belowed things from both old and new node:
- creating UDA with reduce function - not allowed
- selecting count(*) - distributed
- selecting other aggregate function - not distributed
Fixes: #10224Closes#10295
* github.com:scylladb/scylla:
test: add tests for parallelized aggregates
test: cql3: Add UDA REDUCEFUNC test
forward_service: enable multiple selection
forward_service: support UDA and native aggregate parallelization
cql3:functions: Add cql3::functions::functions::mock_get()
cql3: selection: detect parallelize reduction type
db,cql3: Move part of cql3's function into db
selection: detect if selectors factory contains only simple selectors
cql3: reducible aggregates
DB: Add `scylla_aggregates` system table
db,gms: Add SCYLLA_AGGREGATES schema features
CQL3: Add reduce function to UDA
gms: add UDA_NATIVE_PARALLELIZED_AGGREGATION feature
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