Partitioning with Apache Iceberg: A Deep Dive

Published in

Data, Analytics & AI with Dremio

5 min readMay 29, 2024

Introduction

Partitioning is a fundamental concept in data management that significantly enhances query performance by organizing data into distinct segments. This technique groups similar rows together based on specific criteria, making it easier and faster to retrieve relevant data.

Apache Iceberg is an open table format designed for large analytic datasets. It brings high performance and reliability to data lake architectures, offering advanced capabilities such as hidden partitioning, which simplifies data management and improves query efficiency. In this blog, we will explore the partitioning capabilities of Apache Iceberg, highlighting how it stands out from traditional partitioning methods and demonstrating its practical applications using Dremio.

What is Partitioning?

Partitioning is a technique used to enhance the performance of queries by grouping similar rows together when data is written to storage. By organizing data in this manner, it becomes much faster to locate and retrieve specific subsets of data during query execution.

For example, consider a logs table where queries typically include a time range filter, such as retrieving logs between 10 and 12 AM:

SELECT level, message FROM logs
WHERE event_time BETWEEN '2018-12-01 10:00:00' AND '2018-12-01 12:00:00';

Configuring the logs table to partition by the date of event_time groups log events into files based on the event date. Apache Iceberg keeps track of these dates, enabling the query engine to skip over files that do not contain relevant data, thereby speeding up query execution.

Iceberg supports partitioning by various granularities such as year, month, day, and hour. It can also partition data based on categorical columns, such as the level column in the logs example, to further optimize query performance.

Traditional Partitioning Approaches

Traditional table formats like Hive also support partitioning, but they require explicit partitioning columns.

To illustrate the difference between traditional partitioning and Iceberg’s approach, let’s consider how Hive handles partitioning with a sales table.

In Hive, partitions are explicit and must be defined as separate columns. For a sales table, this means creating a sale_date column and manually inserting data into partitions:

INSERT INTO sales PARTITION (sale_date)
  SELECT product_id, amount, sale_time, format_time(sale_time, 'YYYY-MM-dd')
  FROM unstructured_sales_source;

Querying the sales table in Hive also requires an additional filter on the partition column:

SELECT product_id, count(1) as count FROM sales
WHERE sale_time BETWEEN '2022-01-01 10:00:00' AND '2022-01-01 12:00:00'
  AND sale_date = '2022-01-01';

Problems with Hive Partitioning:

Manual Partition Management: Hive requires explicit partition columns and manual insertion of partition values, increasing the likelihood of errors.
Lack of Validation: Hive cannot validate partition values, leading to potential inaccuracies if the wrong format or source column is used.
Query Complexity: Queries must include filters on partition columns to benefit from partitioning, making them more complex and error-prone.
Static Partition Layouts: Changing the partitioning scheme in Hive can break existing queries, limiting flexibility. These issues highlight the challenges of traditional partitioning approaches, which Iceberg overcomes with its automated and hidden partitioning capabilities.

What Does Iceberg Do Differently?

Apache Iceberg addresses the limitations of traditional partitioning by introducing hidden partitioning, which automates and simplifies the partitioning process.

Key Features of Iceberg’s Partitioning:

Hidden Partitioning: Iceberg automatically handles the creation of partition values, removing the need for explicit partition columns. This reduces errors and simplifies data management.
Automatic Partition Pruning: Iceberg can skip unnecessary partitions during query execution without requiring additional filters. This optimization ensures faster query performance.
Evolving Partition Layouts: Iceberg allows partition layouts to evolve over time as data volumes change, without breaking existing queries. This flexibility makes it easier to adapt to changing data requirements.

For example, in an Iceberg table, sales can be partitioned by date and product category without explicitly maintaining these columns:

CREATE TABLE sales (
  product_id STRING,
  amount DECIMAL,
  sale_time TIMESTAMP,
  category STRING
) PARTITIONED BY (date(sale_time), category);

With Iceberg’s hidden partitioning, producers and consumers do not need to be aware of the partitioning scheme, leading to more straightforward and error-free data operations. This approach ensures that partition values are always produced correctly and used to optimize queries.

Iceberg Partition Transformations

Apache Iceberg supports a variety of partition transformations that allow for flexible and efficient data organization. These transformations help optimize query performance by logically grouping data based on specified criteria.

Overview of Supported Partition Transformations

Year, Month, Day, Hour Transformations: These transformations are used for timestamp columns to partition data by specific time intervals.
Categorical Column Transformations:

Bucket: Partitions data by hashing values into a specified number of buckets.
Truncate: Partitions data by truncating values to a specified length, suitable for strings or numeric ranges.

Example Scenarios for Each Transformation

Year, Month, Day, Hour Transformations: Beneficial for time-series data where queries often filter by date ranges. For example, partitioning sales data by month can significantly speed up monthly sales reports.
Bucket Transformation: Useful for columns with high cardinality, such as user IDs, to evenly distribute data across partitions and avoid skew.
Truncate Transformation: Effective for partitioning data with predictable ranges or fixed-length values, such as product codes or zip codes.

Configuring Partitioning in Iceberg

Iceberg makes it straightforward to configure partitions when creating or modifying tables.

Syntax and Examples for Creating Iceberg Tables with Partitions

To create an Iceberg table partitioned by month:

CREATE TABLE sales (
  product_id STRING,
  amount DECIMAL,
  sale_time TIMESTAMP,
  category STRING
) PARTITIONED BY (month(sale_time));

This configuration will group sales records by the month of the sale_time, optimizing queries that filter by month.

Using the ALTER TABLE Command to Modify Partition Schemes Iceberg allows you to modify the partitioning scheme of existing tables using the ALTER TABLE command. For instance, you can add a new partition field:

ALTER TABLE sales ADD PARTITION FIELD year(sale_time);

This command updates the partitioning scheme to include both month(sale_time) and year(sale_time), enhancing query performance for both monthly and yearly aggregations.

Iceberg’s flexible partitioning capabilities, combined with its hidden partitioning feature, ensure that data is always optimally organized for efficient querying and analysis.

Query Optimization with Partitioning

Apache Iceberg leverages its advanced partitioning capabilities to optimize query performance by minimizing the amount of data scanned during query execution. By organizing data into partitions based on specified transformations, Iceberg ensures that only relevant partitions are read, significantly speeding up query response times.

How Iceberg Uses Partitioning to Optimize Queries

Iceberg’s hidden partitioning and automatic partition pruning capabilities enable it to skip over irrelevant data, reducing I/O and improving query performance. When a query is executed, Iceberg uses the partition metadata to determine which partitions contain the data required by the query, thereby avoiding unnecessary scans.