Currently, aggregate functions are implemented in a statefull manner.
The accumulator is stored internally in an aggregate_function::aggregate,
requiring each query to instantiate new instances (see
aggregate_function_selector's constructor, and note how it's called
from selector::new_instance()).
This makes aggregates hard to use in expressions, since expressions
are stateless (with state only provided to evaluate()). To facilitate
migration towards stateless expressions, we define a
stateless_aggregate_function (modeled after user-defined aggregates,
which are already stateless). This new struct defines the aggregate
in terms of three scalar functions: one to aggregate a new input into
an accumulator (provided in the first parameter), one to finalize an
accumulator into a result, and one to reduce two accumulators for
parallelized aggregation.
All existing native aggregate functions are converted to the new model, and
the old interface is removed. This series does not yet convert selectors to
expressions, but it does remove one of the obstacles.
Performance evaluation: I created a table with a million ints on a single-node cluster, and ran the avg() function on them. I measured the number of instructions executed with `perf stat -p $(pgrep scylla) -e instructions` while the query was running. The query executed from cache, memtables were flushed beforehand. The instruction count per row increased from roughly 49k to roughly 52k, indicating 3k extra instructions per row. While 3k instructions to execute a function is huge, it is currently dwarfed by other overhead (and will be even less important in a cluster where it CL>1 will cause non-coordinator code to run multiple times).
Closes#13105
* github.com:scylladb/scylladb:
cql3/selection, forward_service: use use stateless_aggregate_function directly
db: functions: fold stateless_aggregate_function_adapter into aggregate_function
cql3: functions: simplify accumulator_for template
cql3: functions: base user-defined aggregates on stateless aggregates
cql3: functions: drop native_aggregate_function
cql3: functions: reimplement count(column) statelessly
cql3: functions: reimplement avg() statelessly
cql3: functions: reimplement sum() statelessly
cql3: functions: change wide accumulator type to varint
cql3: functions: unreverse types for min/max
cql3: functions: rename make_{min,max}_dynamic_function
cql3: functions: reimplement min/max statelessly
cql3: functions: reimplement count(*) statelessly
cql3: functions: simplify creating native functions even more
cql3: functions: add helpers for automating marshalling for scalar functions
types: fix big_decimal constructor from literal 0
cql3: functions: add helper class for internal scalar functions
db: functions: add stateless aggregate functions
db, cql3: move scalar_function from cql3/functions to db/functions
59ab9aac44