What is Column Pruning and Projection Pushdown in Parquet?

Column pruning (also called projection pushdown) is an optimisation where the query engine reads only the columns referenced in the query, ignoring all other column chunks on disk. Because Parquet stores each column separately, skipping unrequested columns costs nothing beyond reading the footer.

Example — a table has 50 columns but the query only needs 3:

SELECT user_id, revenue, country FROM events WHERE date = '2026-01-01';

The engine reads only the user_id, revenue, country, and date column chunks — the other 46 are never loaded from disk. Combined with predicate pushdown (row group skipping via statistics), this makes Parquet queries orders of magnitude faster than scanning full rows.

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