Generator and Table-Valued Functions (Reference)

Generators are relation-shaping operations. They are registry-backed like scalar and aggregate helpers, but they return GeneratorApplication values and must be applied through a relation method such as generate(...).

The explicit generator surface currently includes:

Function	Output aliases	Relation effect
explode(expr, as_)	one value column	Emits one row per array element; null or empty inputs emit zero rows.
explode_outer(expr, as_)	one value column	Preserves the input row for null or empty inputs and emits a null generated value.
posexplode(expr, position_as, value_as)	position and value columns	Emits one row per array element with a zero-based position column.
posexplode_outer(expr, position_as, value_as)	position and value columns	Outer positional explode with the same zero-based position rule.
inline(expr, output_columns)	one column per struct field	Expands array-of-struct values into generated rows and declared output columns.
inline_outer(expr, output_columns)	one column per struct field	Outer inline with the same null/empty row preservation rule.
flatten(expr, as_)	one value column	Portable table-valued flatten for one array expression.
stack(row_count, values, output_columns)	declared output columns	Emits row_count generated rows from row-major scalar values.

Generator applications preserve input columns and append generated columns in declaration order. Generated aliases are required, must be non-empty, and must not collide with existing input columns.

Nested scalar helpers such as array_flatten(...) remain scalar expressions. They do not expand rows and are documented on the nested data functions page. The relation-shaping flatten(...) helper is intentionally separate.

For task-oriented usage, see Expand rows with generators.