Register DISTINCT_COUNT_APPROX logical marker in PPLFuncImpTable

[songkant-aws]

Description

PPL parser fails with Cannot resolve function: DISTINCT_COUNT_APPROX on any execution path that does not construct OpenSearchExecutionEngine (unified-query / analytics-engine / DataFusion), because the UDAF was only registered to PPLFuncImpTable.aggExternalFunctionRegistry as a side effect of that constructor (registerOpenSearchFunctions).

This change adds a logical marker UDAF in core that lets PPL parser succeed on every path. Backends are expected to push down or rewrite the operator before execution.

Layered registration

• core (this PR): register a marker DistinctCountApproxLogicalAggFunction in PPLFuncImpTable.AggBuilder.populate() via the new PPLBuiltinOperators.DISTINCT_COUNT_APPROX. Lookup precedence in PPLFuncImpTable.getImplementation() is aggExternalFunctionRegistry first, then aggFunctionRegistry.
• opensearch (unchanged): OpenSearchExecutionEngine.registerOpenSearchFunctions() still installs the real DistinctCountApproxAggFunction (HyperLogLog++) into aggExternalFunctionRegistry. Because external is consulted first, the V3 path always sees the real implementation once the constructor has run, and the marker is effectively overridden.

This is the same pattern already used by RelevanceQueryFunction.RelevanceQueryImplementor for match/match_phrase/etc — relevance functions register a no-op operator whose RelevanceQueryImplementor.implement() throws UnsupportedOperationException, because their semantics live entirely on the OpenSearch index side. DISTINCT_COUNT_APPROX is in the same situation: real semantics on the OpenSearch side (cardinality aggregation) and on backends like DataFusion (approx_count_distinct).

Marker class behavior

init / add / result and the accumulator's value all throw UnsupportedOperationException with the message:

DISTINCT_COUNT_APPROX logical marker reached Enumerable execution; an engine-specific implementation must be registered or rewritten before execution.

Reaching the body means a backend either failed to push down or did not register an adapter — surfacing this as a clear error rather than silently producing wrong results is the intended behavior.

V3 path behavior is preserved

• AggregateAnalyzer translates DISTINCT_COUNT_APPROX to OpenSearch cardinality aggregation through a BuiltinFunctionName switch, independent of the wrapped SqlAggFunction instance — unchanged by this PR.
• When V3 falls back to Calcite enumerable execution (push-down failed for some sub-plan), RexImpTable.getAggImplementor reads SqlUserDefinedAggFunction.function, which is the real DistinctCountApproxAggFunction registered by OpenSearchExecutionEngine (external takes precedence) — unchanged.
• Operand metadata for the marker is null to match the existing external registration's permissive type policy. No new type rejection is introduced.

How this unblocks unified-query / DataFusion path

RestUnifiedQueryAction does not construct OpenSearchExecutionEngine; before this change, dc() / distinct_count() / distinct_count_approx() failed at PPL parse stage on that path. After this change, the parser succeeds via the marker, and the downstream backend (e.g. analytics-engine's DataFusion adapter) rewrites RexOver(DISTINCT_COUNT_APPROX) to APPROX_COUNT_DISTINCT before substrait emission. End-to-end verified locally with the analytics-engine REST IT in the OpenSearch sandbox.

Related Issues

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
Possible issue	Replace null operand checker Passing null for operand type checker can cause NullPointerException during query validation if Calcite attempts to validate operands before backend rewriting occurs. Consider using PPLOperandTypes.ANY or a permissive operand checker instead of null to ensure safe validation behavior. core/src/main/java/org/opensearch/sql/expression/function/PPLBuiltinOperators.java [521-526] public static final SqlAggFunction DISTINCT_COUNT_APPROX = createUserDefinedAggFunction( DistinctCountApproxLogicalAggFunction.class, "DISTINCT_COUNT_APPROX", ReturnTypes.BIGINT_FORCE_NULLABLE, - null); + PPLOperandTypes.ANY); Suggestion importance[1-10]: 7 __ Why: Valid concern about potential NullPointerException during query validation. The comment explicitly states null matches existing external registration's permissive policy, but using PPLOperandTypes.ANY would be safer while maintaining permissiveness. However, the suggestion assumes validation occurs before backend rewriting without confirming the actual execution flow.	Medium