Mechanism for foldable aggregations in ES|QL

[astefan]

Description

Currently, ES|QL lacks two major performance improvements aspects for aggregations (stats):

• planner (LogicalPlanOptimizer component) propagation of foldable expressions into aggregations. This is mainly about extending the functionality in PropagateEvalFoldables optimization rule.
• short-circuiting null handling in aggregation functions so that aggregations on null constant do not reach the compute engine and is already computed locally. There are multiple already created issues about this.

A first attempt at addressing this introduced too many workarounds to be considered valuable and merge-worthy:

• Calling SubstituteSurrogates twice. The SubstituteSurrogates rule should be called only once, its role shouldn't be a partial mechanism for dealing with constants in aggregate functions, as it happens now.
• surrogate expression replacement before the entire set of constants have been folded and propagated can introduce issues for validation of arguments correctness. See this situation where currently this is an issue. More about this concept here and here as well.
• having SubstituteSurrogates be called twice introduced yet another issue and broke existent working behavior

As a consequence, aggregations must support a specific mechanism in which they can provide a result given they receive constants as arguments, probably in the form of an interface. This should also come with support in the LogicalPlanOptimizer in the form of a new rule + changes to existent rules that deal with folding, null folding and constants propagation.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)