Support aggregates with stype = internal

[ChenHuajun]

Now, aggregation functions that support parallel execution in Citus are configured through a hard-coded whitelist in multi_logical_optimizer.h.

We need to use the aggregate function in pg_roaringbitmap

https://github.com/ChenHuajun/pg_roaringbitmap
-rb_and_agg
-rb_or_agg

Could this simple whitelist of aggregation functions satisfying the associativity be made configurable?

[metdos]

@ChenHuajun I suppose you don't add distribution column to your queries?
https://docs.citusdata.com/en/v9.3/develop/reference_sql.html?highlight=coordinator_aggregation_strategy#aggregate-functions

[onurctirtir]

Citus actually supports pushing execution to the worker nodes for custom aggregates but it does not support it for aggregates having their state_data_type to be internal

[ChenHuajun]

@ChenHuajun I suppose you don't add distribution column to your queries?
https://docs.citusdata.com/en/v9.3/develop/reference_sql.html?highlight=coordinator_aggregation_strategy#aggregate-functions

Yes. As @onurctirtir said what i need is support of aggregates with stype = internal

[marcocitus]

More context in #120

We should add a metadata table that contains "safe" aggregates (and check it in AggregateEnabledCustom) and then implement serialization in worker_partial_agg_ffunc and deserialization in coord_combine_agg_sfunc.