Improve error message for unsupported window functions in eventstats/streamstats

[gingeekrishna]

Description

Fixes #5168

eventstats/streamstats reject window functions outside WINDOW_FUNC_MAPPING (e.g. rank(), dense_rank(), nth_value()) with a bare "Unexpected window function: X" error.

As confirmed in the issue discussion by @songkant-aws, this is expected behavior, not a bug — rank/dense_rank/nth_value require ORDER BY semantics, but eventstats/streamstats only support partition by. The issue was relabeled error-experience: the fix is to make the error message clearer, not to add support for these functions.

Changes

• CalciteRexNodeVisitor#visitWindowFunction: replaced the generic "Unexpected window function: X" message with one that names the rejected function and lists the functions eventstats/streamstats do support (sourced from WINDOW_FUNC_MAPPING), so users get actionable guidance instead of a bare error.
• Updated the existing unit test (UnifiedQueryPlannerTest) and integration tests (CalcitePPLEventstatsIT, CalciteStreamstatsCommandIT) to assert against the new message.
• Added new integration tests covering rank/dense_rank specifically, since the issue's repro queries used those functions.

Note: this PR builds on top of #5587 (already merged), which fixed the same throw site to return a 4xx instead of a 500. This PR only changes the message text, not the exception type or HTTP status.

Test plan

• Unit test UnifiedQueryPlannerTest#unsupportedWindowFunctionIsRethrownAsSemanticCheckException passes
• Integration tests added/updated for eventstats and streamstats (require a live cluster to run in CI)

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PR Reviewer Guide 🔍

(Review updated until commit 99c53f5)

Here are some key observations to aid the review process:

🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ Recommended focus areas for review Hardcoded list The error message hardcodes the list of supported functions as a string literal. If WINDOW_FUNC_MAPPING is updated to add or remove functions, this message will become stale and misleading. The list should be generated dynamically from WINDOW_FUNC_MAPPING.keySet() to stay in sync. () -> new CalciteUnsupportedException( "Window function '" + funcName + "' is not supported in eventstats/streamstats." + " Supported functions: avg, count, dc, distinct_count, earliest," + " latest, max, min, row_number, stddev_pop, stddev_samp, sum," + " var_pop, var_samp."));

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PR Code Suggestions ✨

Latest suggestions up to 99c53f5
Explore these optional code suggestions:

Category	Suggestion	Impact
General	Use String.format for error message The error message concatenates multiple strings which can impact readability and maintainability. Consider using String.format() or a text block for better formatting and easier future modifications of the supported functions list. core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java [709-715] new CalciteUnsupportedException( - "Window function '" - + funcName - + "' is not supported in eventstats/streamstats." - + " Supported functions: avg, count, dc, distinct_count, earliest," - + " latest, max, min, row_number, stddev_pop, stddev_samp, sum," - + " var_pop, var_samp.")); + String.format( + "Window function '%s' is not supported in eventstats/streamstats." + + " Supported functions: avg, count, dc, distinct_count, earliest," + + " latest, max, min, row_number, stddev_pop, stddev_samp, sum," + + " var_pop, var_samp.", + funcName)); Suggestion importance[1-10]: 4 __ Why: While using String.format() can improve readability, the current string concatenation approach is perfectly valid and readable. This is a minor style improvement that doesn't significantly impact code quality or functionality.	Low

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Previous suggestions

Suggestions up to commit c16fc0c

Category	Suggestion	Impact
General	Dynamically generate supported functions list The hardcoded list of supported functions in the error message may become outdated if new functions are added to WINDOW_FUNC_MAPPING. Consider dynamically generating this list from the mapping keys to ensure accuracy and maintainability. core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java [709-715] new CalciteUnsupportedException( "Window function '" + funcName + "' is not supported in eventstats/streamstats." - + " Supported functions: avg, count, dc, distinct_count, earliest," - + " latest, max, min, row_number, stddev_pop, stddev_samp, sum," - + " var_pop, var_samp.")); + + " Supported functions: " + + String.join(", ", WINDOW_FUNC_MAPPING.keySet()) + + ".")); Suggestion importance[1-10]: 7 __ Why: This suggestion improves maintainability by dynamically generating the list of supported functions from WINDOW_FUNC_MAPPING.keySet() instead of hardcoding them. This ensures the error message stays accurate when new functions are added, reducing maintenance burden and preventing outdated documentation.	Medium

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Persistent review updated to latest commit 99c53f5