Map Calcite ROW to STRUCT in convertSqlTypeNameToExprType

[mch2]

Description

convertExprTypeToRelDataType builds STRUCT as MAP<VARCHAR, ANY>, since the v2 path only passes _source JSON through and needs no field types. So the reverse mapping only had case MAP -> STRUCT — a genuine Calcite ROW matched nothing, fell through to default -> UNKNOWN, and surfaced as "type": "unknown" in the response schema.

That is reachable from the analytics engine, which materializes an object field as a real ROW built from typed columns. Before, fields city reported "unknown"; now "struct", matching what a lucene-only cluster returns for the same mapping.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

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]

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Category	Suggestion	Impact
General	Document conversion asymmetry issue The comment mentions that convertExprTypeToRelDataType builds STRUCT as MAP<VARCHAR, ANY>, but this creates an inconsistency. When ROW is converted to STRUCT here, and then STRUCT is converted back via convertExprTypeToRelDataType, it becomes a MAP instead of a ROW, breaking round-trip conversion. Consider documenting this asymmetry or ensuring bidirectional consistency. core/src/main/java/org/opensearch/sql/calcite/utils/OpenSearchTypeFactory.java [255-259] // Calcite spells a struct as ROW. convertExprTypeToRelDataType builds STRUCT as // MAP<VARCHAR, ANY> since the v2 path only passes _source JSON through, so a genuine ROW — // from an engine that builds a struct out of typed columns — matched nothing and fell // through to UNKNOWN. +// Note: This creates an asymmetry where ROW->STRUCT->MAP in round-trip conversions. case ROW -> STRUCT; Suggestion importance[1-10]: 4 __ Why: The suggestion correctly identifies a potential round-trip conversion asymmetry between ROW->STRUCT->MAP. However, it only proposes adding a comment rather than fixing the underlying issue, and the existing comment already explains the context. The added note provides marginal value.	Low