---
title: "Zone map (min-max index)"
description: "A zone map, or min-max index, is a lightweight index that stores the minimum and maximum value of a column for each block of data, letting queries skip blocks that can't match a filter."
canonical: "https://motherduck.com/glossary/zone-map/"
related:
  - title: "DuckLake Architecture Deep Dive"
    url: "https://motherduck.com/blog/ducklake-architecture-deep-dive/"
  - title: "CREATE INDEX | MotherDuck Docs"
    url: "https://motherduck.com/docs/sql-reference/duckdb-sql-reference/duckdb-statements/create-index/"
  - title: "Window functions | MotherDuck Docs"
    url: "https://motherduck.com/docs/sql-reference/duckdb-sql-reference/window-functions/"
---

# Zone map (min-max index)

> A zone map, or min-max index, is a lightweight index that stores the minimum and maximum value of a column for each block of data, letting queries skip blocks that can't match a filter.

## Overview

A zone map (also called a block range index or min-max index) is one of the cheapest and most effective pruning techniques in analytical databases. For each contiguous block of rows, the database records the minimum and maximum value of a column within that block. When a query filters on that column, the engine checks each block's min-max range against the filter and skips any block whose range can't possibly contain a match — without reading the block's actual data at all.

<glossary-callout guide="duckdb-cheatsheet-full" />

## How Zone Maps Work

Given a filter like `WHERE order_date = '2024-06-01'`, a block whose recorded range is `2023-01-01` to `2023-12-31` can be skipped immediately, since the filter value falls entirely outside that range. Zone maps are cheap to build and store — just two values per block per column — and cheap to check, but they only help when data has some locality: the more a column is sorted or naturally clustered within blocks, the more values zone maps can prune.

## Zone Maps vs. Traditional Indexes

Unlike a B-tree index, which points to exact row locations, a zone map only tells you whether a *block* might contain a match. It can't tell you which row within the block, so it's purely a pruning device that reduces I/O — the block still has to be scanned if it passes the min-max check.

## DuckDB's Zone Maps

DuckDB automatically maintains a min-max index for every column of every general-purpose data type, for each row group, with no manual index creation required. Combined with predicate pushdown, this lets DuckDB skip entire row groups — whether stored natively or in Parquet files — that can't satisfy a query's `WHERE` clause. Because these zone maps are most effective on ordered data, loading data pre-sorted on frequently filtered columns can substantially improve how much a query can prune.