docs/source/user-guide/sql/struct_coercion.md

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# Struct Type Coercion and Field Mapping

DataFusion uses **name-based field mapping** when coercing struct types across different operations. This document explains how struct coercion works, when it applies, and how to handle NULL fields.

## Overview: Name-Based vs Positional Mapping

When combining structs from different sources (e.g., in UNION, array construction, or JOINs), DataFusion matches struct fields by **name** rather than by **position**. This provides more robust and predictable behavior compared to positional matching.

### Example: Field Reordering is Handled Transparently

```sql
-- These two structs have the same fields in different order
SELECT [{a: 1, b: 2}, {b: 3, a: 4}];

-- Result: Field names matched, values unified
-- [{"a": 1, "b": 2}, {"a": 4, "b": 3}]
```

## Coercion Paths Using Name-Based Matching

The following query operations use name-based field mapping for struct coercion:

### 1. Array Literal Construction

When creating array literals with struct elements that have different field orders:

```sql
-- Structs with reordered fields in array literal
SELECT [{x: 1, y: 2}, {y: 3, x: 4}];

-- Unified type: List(Struct("x": Int32, "y": Int32))
-- Values: [{"x": 1, "y": 2}, {"x": 4, "y": 3}]
```

**When it applies:**

- Array literals with struct elements: `[{...}, {...}]`
- Nested arrays with structs: `[[{x: 1}, {x: 2}]]`

### 2. Array Construction from Columns

When constructing arrays from table columns with different struct schemas:

```sql
CREATE TABLE t_left (s struct(x int, y int)) AS VALUES ({x: 1, y: 2});
CREATE TABLE t_right (s struct(y int, x int)) AS VALUES ({y: 3, x: 4});

-- Dynamically constructs unified array schema
SELECT [t_left.s, t_right.s] FROM t_left JOIN t_right;

-- Result: [{"x": 1, "y": 2}, {"x": 4, "y": 3}]
```

**When it applies:**

- Array construction with column references: `[col1, col2]`
- Array construction in joins with matching field names

### 3. UNION Operations

When combining query results with different struct field orders:

```sql
SELECT {a: 1, b: 2} as s
UNION ALL
SELECT {b: 3, a: 4} as s;

-- Result: {"a": 1, "b": 2} and {"a": 4, "b": 3}
```

**When it applies:**

- UNION ALL with structs: field names matched across branches
- UNION (deduplicated) with structs

### 4. Common Table Expressions (CTEs)

When multiple CTEs produce structs with different field orders that are combined:

```sql
WITH
  t1 AS (SELECT {a: 1, b: 2} as s),
  t2 AS (SELECT {b: 3, a: 4} as s)
SELECT s FROM t1
UNION ALL
SELECT s FROM t2;

-- Result: Field names matched across CTEs
```

### 5. VALUES Clauses

When creating tables or temporary results with struct values in different field orders:

```sql
CREATE TABLE t AS VALUES ({a: 1, b: 2}), ({b: 3, a: 4});

-- Table schema unified: struct(a: int, b: int)
-- Values: {a: 1, b: 2} and {a: 4, b: 3}
```

### 6. JOIN Operations

When joining tables where the JOIN condition involves structs with different field orders:

```sql
CREATE TABLE orders (customer struct(name varchar, id int));
CREATE TABLE customers (info struct(id int, name varchar));

-- Join matches struct fields by name
SELECT * FROM orders
JOIN customers ON orders.customer = customers.info;
```

### 7. Aggregate Functions

When collecting structs with different field orders using aggregate functions like `array_agg`:

```sql
SELECT array_agg(s) FROM (
  SELECT {x: 1, y: 2} as s
  UNION ALL
  SELECT {y: 3, x: 4} as s
) t
GROUP BY category;

-- Result: Array of structs with unified field order
```

### 8. Window Functions

When using window functions with struct expressions having different field orders:

```sql
SELECT
  id,
  row_number() over (partition by s order by id) as rn
FROM (
  SELECT {category: 1, value: 10} as s, 1 as id
  UNION ALL
  SELECT {value: 20, category: 1} as s, 2 as id
);

-- Fields matched by name in PARTITION BY clause
```

## NULL Handling for Missing Fields

When structs have different field sets, missing fields are filled with **NULL** values during coercion.

### Example: Partial Field Overlap

```sql
-- Struct in first position has fields: a, b
-- Struct in second position has fields: b, c
-- Unified schema includes all fields: a, b, c

SELECT [
  CAST({a: 1, b: 2} AS STRUCT(a INT, b INT, c INT)),
  CAST({b: 3, c: 4} AS STRUCT(a INT, b INT, c INT))
];

-- Result:
-- [
--   {"a": 1, "b": 2, "c": NULL},
--   {"a": NULL, "b": 3, "c": 4}
-- ]
```

### Limitations

**Field count must match exactly.** If structs have different numbers of fields and their field names don't completely overlap, the query will fail:

```sql
-- This fails because field sets don't match:
-- t_left has {x, y} but t_right has {x, y, z}
SELECT [t_left.s, t_right.s] FROM t_left JOIN t_right;
-- Error: Cannot coerce struct with mismatched field counts
```

**Workaround: Use explicit CAST**

To handle partial field overlap, explicitly cast structs to a unified schema:

```sql
SELECT [
  CAST(t_left.s AS STRUCT(x INT, y INT, z INT)),
  CAST(t_right.s AS STRUCT(x INT, y INT, z INT))
] FROM t_left JOIN t_right;
```

## Migration Guide: From Positional to Name-Based Matching

If you have existing code that relied on **positional** struct field matching, you may need to update it.

### Example: Query That Changes Behavior

**Old behavior (positional):**

```sql
-- These would have been positionally mapped (left-to-right)
SELECT [{x: 1, y: 2}, {y: 3, x: 4}];
-- Old result (positional): [{"x": 1, "y": 2}, {"y": 3, "x": 4}]
```

**New behavior (name-based):**

```sql
-- Now uses name-based matching
SELECT [{x: 1, y: 2}, {y: 3, x: 4}];
-- New result (by name): [{"x": 1, "y": 2}, {"x": 4, "y": 3}]
```

### Migration Steps

1. **Review struct operations** - Look for queries that combine structs from different sources
2. **Check field names** - Verify that field names match as expected (not positions)
3. **Test with new coercion** - Run queries and verify the results match your expectations
4. **Handle field reordering** - If you need specific field orders, use explicit CAST operations

### Using Explicit CAST for Compatibility

If you need precise control over struct field order and types, use explicit `CAST`:

```sql
-- Guarantee specific field order and types
SELECT CAST({b: 3, a: 4} AS STRUCT(a INT, b INT));
-- Result: {"a": 4, "b": 3}
```

## Best Practices

### 1. Be Explicit with Schema Definitions

When joining or combining structs, define target schemas explicitly:

```sql
-- Good: explicit schema definition
SELECT CAST(data AS STRUCT(id INT, name VARCHAR, active BOOLEAN))
FROM external_source;
```

### 2. Use Named Struct Constructors

Prefer named struct constructors for clarity:

```sql
-- Good: field names are explicit
SELECT named_struct('id', 1, 'name', 'Alice', 'active', true);

-- Or using struct literal syntax
SELECT {id: 1, name: 'Alice', active: true};
```

### 3. Test Field Mappings

Always verify that field mappings work as expected:

```sql
-- Use arrow_typeof to verify unified schema
SELECT arrow_typeof([{x: 1, y: 2}, {y: 3, x: 4}]);
-- Result: List(Struct("x": Int32, "y": Int32))
```

### 4. Handle Partial Field Overlap Explicitly

When combining structs with partial field overlap, use explicit CAST:

```sql
-- Instead of relying on implicit coercion
SELECT [
  CAST(left_struct AS STRUCT(x INT, y INT, z INT)),
  CAST(right_struct AS STRUCT(x INT, y INT, z INT))
];
```

### 5. Document Struct Schemas

In complex queries, document the expected struct schemas:

```sql
-- Expected schema: {customer_id: INT, name: VARCHAR, age: INT}
SELECT {
  customer_id: c.id,
  name: c.name,
  age: c.age
} as customer_info
FROM customers c;
```

## Error Messages and Troubleshooting

### "Cannot coerce struct with different field counts"

**Cause:** Trying to combine structs with different numbers of fields.

**Solution:**

```sql
-- Use explicit CAST to handle missing fields
SELECT [
  CAST(struct1 AS STRUCT(a INT, b INT, c INT)),
  CAST(struct2 AS STRUCT(a INT, b INT, c INT))
];
```

### "Field X not found in struct"

**Cause:** Referencing a field name that doesn't exist in the struct.

**Solution:**

```sql
-- Verify field names match exactly (case-sensitive)
SELECT s['field_name'] FROM my_table;  -- Use bracket notation for access
-- Or use get_field function
SELECT get_field(s, 'field_name') FROM my_table;
```

### Unexpected NULL values after coercion

**Cause:** Struct coercion added NULL for missing fields.

**Solution:** Check that all structs have the required fields, or explicitly handle NULLs:

```sql
SELECT COALESCE(s['field'], default_value) FROM my_table;
```

## Related Functions

- `arrow_typeof()` - Returns the Arrow type of an expression
- `struct()` / `named_struct()` - Creates struct values
- `get_field()` - Extracts field values from structs
- `CAST()` - Explicitly casts structs to specific schemas