babelfish-for-postgresql / babelfish_extensions / 24984890051

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Fix sql_variant type resolution in CASE, UNION, INTERSECT, EXCEPT, VALUES, and COALESCE expressions (#4706)

Expressions fail when sql_variant is mixed with other types in type-resolution contexts:

-- CASE: fails
SELECT CASE WHEN 1 = 1 THEN SERVERPROPERTY('IsFullTextInstalled') ELSE CAST(0 AS bit) END;
-- Error: CASE/WHEN could not convert type sql_variant to "bit"
-- COALESCE: fails
SELECT COALESCE(CAST(NULL AS sql_variant), CAST('hello' AS varchar(50)));
-- Error: COALESCE could not convert type sql_variant to "varchar"
The same class of issue affects UNION, INTERSECT, EXCEPT, and VALUES when sql_variant is mixed with non-char types.

Fix
select_common_type_setop - Added a pre-scan before the main loop: if any expression is sql_variant, return it immediately as the common type. This applies to all contexts routed through this function (CASE, UNION, INTERSECT, EXCEPT, VALUES). sql_variant has the highest TSQL precedence (0) and can hold any type, so it always wins.

select_common_type_for_coalesce_function - In the NULL-skipping logic, if the NULL arg's type is sql_variant, still consider it for precedence before continuing.

Both fixes use is_tsql_sqlvariant_datatype to narrowly target only sql_variant. No other types or code paths are affected.

Issues Resolved
BABEL-6402
BABEL-6427

Signed-off by: Mohit Raj mrxmohit@amazon.com

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Co-authored-by: Mohit Raj <mrxmohit@amazon.com>

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