[IGNITE-21954] Reuse Calcite map-reduce aggregates code.

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: sql
• Labels: ignite-3 sql tech-debt
• Ignite Flags: Docs Required, Release Notes Required
• Assignee: Unassigned
• Reporter: Andrey Mashenkov
• Created: 03/Apr/24 13:24
• Updated: 16/Apr/24 13:37

Description

Motivation

As for now we have custom code that splits aggregate function into map-reduce stages. See class/method usage for details:

 

MapReduceAggregates.buildAggregates

 

Some functions (e.g. AVG) are split into 2 functions which may have incompatible types (e.g. COUNT returns BIGINT on map-phase, but SUM on reduce-phase requires DECIMAL).
This forces adding a Projection in between, but this additional stage looks synthetic and rises costs.

I've found Calcite already have `AggregateReduceFunctionsRule`, which splits functions in different way (using `CASE-WHEN` clause instead of Project), and can be reused.

Suggestion

Let's try to reuse Calcite rule and/or splitting functions, and drop duplicating code from our codebase.
Otherwise, if there are obstacles, go the similar way at least.

Fix planner tests, which expects a Project between map and reduce phases.

Expectation

After this fix, we should observe no projection in between map and reduce aggregate phases.

Issue Links

• is caused by: IGNITE-20009 Sql. Rework 2-phase aggregates part 2. AVG as SUM / COUNT.
• is related to: IGNITE-21838 Sql. Provide correct type information for intermediate results of MAP aggregates
• relates to: IGNITE-21580 Sql. Unable to optimise query using only two phase aggregates