reduce_search_phase_exception: reduce_search_phase_exception

What does it mean?

An error occurred in the reduce phase of a search. The coordinating node hit a problem while merging (reducing) partial results/aggregations from different shards and could not produce the final result.

Why does it happen?

Usually a memory/resource limit is exceeded while merging results (e.g. very large aggregations, a circuit breaker tripping), an incompatible/invalid aggregation structure, or a partial result returned by a shard cannot be merged. Common with very high-cardinality aggregations.

How to fix it

Read the caused_by detail to identify the root cause. Lighten heavy aggregations: lower size on high-cardinality terms aggregations, paginate with a composite aggregation, and reduce unnecessary sub-aggregations. Monitor memory pressure and split the query into smaller parts if needed. If a circuit breaker trips, reduce the query cost.