How does Parquet support nested data (structs, lists, maps)?

Parquet uses the Dremel encoding (Google's paper, 2010) to represent arbitrarily nested data in a flat columnar layout. Two extra per-value integers are stored alongside each column's data:

• Definition level — how many optional fields in the path are actually defined (non-null). Encodes null positions without storing nulls explicitly.
• Repetition level — which repeated field in the path started a new list element. Encodes list boundaries.

Example — a column orders.items.price where items is a repeated field:

Row 1: orders.items = [{price: 10}, {price: 20}]
Row 2: orders.items = []  (empty list)

Stored as:
  price column: [10, 20]
  repetition: [0, 1]   (0=new top-level record, 1=repeated value)
  definition: [2, 2, 1] (includes row 2's empty-list marker)

This lets engines reconstruct nested structures perfectly while retaining columnar access for analytics on leaf fields.

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