Fix critical struct converter and JSON syntax issues

- Fixed incorrect to_json() usage: reverted to proper CAST(...AS JSON) syntax per AWS docs
- Fixed struct converter to skip problematic pairs instead of failing completely
- Changed from 'return None' to 'continue' when encountering special characters
- This ensures {a=1, b=2} parses both pairs instead of failing on first issue

Key improvements:
- Partial parsing is now possible (better than complete failure)
- JSON conversion uses official AWS Athena CAST AS JSON syntax
- More resilient struct parsing that handles mixed valid/invalid pairs

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: laughingman7743

pyathena/converter.py

@@ -141,25 +141,25 @@ def _to_struct(varchar_value: Optional[str]) -> Optional[Dict[str, Any]]:
                         value = inner[eq_pos + 1 : comma_pos].strip()
                         current_pos = comma_pos + 1
 
-                    # Basic validation: reject if key or value contains truly problematic chars
+                    # Basic validation: skip problematic pairs but continue processing others
                     # Allow basic comma separation, but reject nested structures
                     if any(char in key for char in '{}="') or any(char in value for char in '{}="'):
-                        # Fall back to returning the original string for complex cases
-                        return None
-
-                # Add quotes to key
-                key = f'"{key}"'
-
-                # Handle value quoting - if it's not a number, quote it
-                if not (
-                    value.isdigit()
-                    or (value.startswith("-") and value[1:].isdigit())
-                    or value.replace(".", "", 1).isdigit()
-                    or value in ("true", "false", "null")
-                ):
-                    value = f'"{value}"'
-
-                pairs.append(f"{key}:{value}")
+                        # Skip this problematic pair but continue with others
+                        continue
+
+                    # Add quotes to key
+                    key = f'"{key}"'
+
+                    # Handle value quoting - if it's not a number, quote it
+                    if not (
+                        value.isdigit()
+                        or (value.startswith("-") and value[1:].isdigit())
+                        or value.replace(".", "", 1).isdigit()
+                        or value in ("true", "false", "null")
+                    ):
+                        value = f'"{value}"'
+
+                    pairs.append(f"{key}:{value}")
 
                 if pairs:
                     json_str = "{" + ",".join(pairs) + "}"

tests/pyathena/test_cursor.py

@@ -779,8 +779,8 @@ def test_struct_types(self, cursor):
                 "SELECT ROW(ROW('John', 30), ROW('Engineer', 'Tech')) AS nested_struct",
                 "nested_struct",
             ),
-            # Struct as JSON (using to_json function)
-            ("SELECT to_json(ROW('Alice', 25, 'Hello, world')) AS json_struct", "json_struct"),
+            # Struct as JSON (using CAST AS JSON)
+            ("SELECT CAST(ROW('Alice', 25, 'Hello, world') AS JSON) AS json_struct", "json_struct"),
         ]
 
         _logger.info("=== STRUCT Type Test Results ===")
@@ -828,8 +828,8 @@ def test_array_types(self, cursor):
             ("SELECT ARRAY[ROW('Alice', 25), ROW('Bob', 30)] AS struct_array", "struct_array"),
             # Nested arrays
             ("SELECT ARRAY[ARRAY[1, 2], ARRAY[3, 4]] AS nested_array", "nested_array"),
-            # Array as JSON (using to_json function instead of CAST)
-            ("SELECT to_json(ARRAY['Alice', 'Bob', 'Charlie']) AS json_array", "json_array"),
+            # Array as JSON (using CAST AS JSON)
+            ("SELECT CAST(ARRAY['Alice', 'Bob', 'Charlie'] AS JSON) AS json_array", "json_array"),
         ]
 
         _logger.info("=== ARRAY Type Test Results ===")
@@ -875,9 +875,9 @@ def test_map_types(self, cursor):
                 ),
                 "struct_value_map",
             ),
-            # Map as JSON (using to_json function)
+            # Map as JSON (using CAST AS JSON)
             (
-                "SELECT to_json(MAP(ARRAY['name', 'age'], ARRAY['Alice', '25'])) AS json_map",
+                "SELECT CAST(MAP(ARRAY['name', 'age'], ARRAY['Alice', '25']) AS JSON) AS json_map",
                 "json_map",
             ),
         ]