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Writing to Iceberg

The iceberg extension supports writing to Iceberg tables that are managed by an Iceberg REST Catalog. All write operations go through the attached catalog and are committed as new Iceberg snapshots.

Writing requires an attached catalog. The path-based iceberg_scan interface described in the overview is read-only. To write, first attach an Iceberg REST catalog.

The examples below assume a catalog has been attached as my_catalog.

Creating Schemas and Tables

Iceberg namespaces are exposed as schemas. You can create and drop schemas and tables with standard SQL:

CREATE SCHEMA my_catalog.sales;
USE my_catalog.sales;

CREATE TABLE my_catalog.sales.events (
    id INTEGER,
    event_name VARCHAR,
    event_time TIMESTAMP
);

-- Create a table from a query
CREATE TABLE my_catalog.sales.events_copy AS
    FROM my_catalog.sales.events;

DROP TABLE my_catalog.sales.events_copy;

Partitioning

Tables can be partitioned with the PARTITIONED BY clause using the Iceberg partition transforms:

Transform Description
column Identity – partition by the column value directly.
year(column), month(column), day(column), hour(column) Partition by a date/timestamp component.
bucket(n, column) Hash the column into n buckets.
truncate(n, column) Truncate the column value to width n. 
CREATE TABLE my_catalog.sales.events (
    id INTEGER,
    event_name VARCHAR,
    event_time TIMESTAMP
)
PARTITIONED BY (day(event_time), bucket(16, id));

The partition spec can be changed on an existing table with ALTER TABLE ... SET PARTITIONED BY:

ALTER TABLE my_catalog.sales.events SET PARTITIONED BY (month(event_time));

The write.target-file-size-bytes and write.parquet.row-group-size-bytes table properties are not honored for partitioned tables and raise an error. Set ignore_target_file_size_for_partitioned_tables or ignore_row_group_size_for_partitioned_tables to true to ignore them instead.

Table Properties

Iceberg table properties can be set at creation time with a WITH clause. The format-version and location keys are recognized specially; any other key-value pairs are stored as table properties:

CREATE TABLE my_catalog.sales.events (a INTEGER)
WITH (
    'format-version' = '2',                 -- Iceberg format version (2 or 3)
    'location' = 's3://my-bucket/events',   -- base location for the table's data
    'my.custom.property' = 'value'
);

Existing properties can be inspected and modified with the property functions:

-- View properties
SELECT * FROM iceberg_table_properties(my_catalog.sales.events);

-- Set properties
CALL set_iceberg_table_properties(
    my_catalog.sales.events,
    MAP {'write.update.mode': 'merge-on-read', 'write.delete.mode': 'merge-on-read'}
);

-- Remove properties
CALL remove_iceberg_table_properties(my_catalog.sales.events, ['my.custom.property']);

See the Functions and Settings Reference for the equivalent schema (namespace) property functions.

Inserting Data 

INSERT INTO my_catalog.sales.events
VALUES (1, 'click', TIMESTAMP '2026-06-01 10:00:00');

-- Insert the result of a query
INSERT INTO my_catalog.sales.events
    SELECT * FROM source_table;

-- Match columns by name rather than position
INSERT INTO my_catalog.sales.events BY NAME
    SELECT event_time, id, event_name FROM source_table;

Updating and Deleting 

UPDATE my_catalog.sales.events SET event_name = 'view' WHERE id = 1;

DELETE FROM my_catalog.sales.events WHERE event_time < TIMESTAMP '2026-01-01';

UPDATE and DELETE are supported on both partitioned and unpartitioned tables. They use merge-on-read semantics and write positional delete files; see Limitations.

Merging Data

MERGE INTO performs an upsert against a source relation. The join key is given with a second USING clause (see the MERGE INTO statement):

MERGE INTO my_catalog.sales.events AS target
USING new_events AS source USING (id)
WHEN MATCHED THEN UPDATE SET event_name = source.event_name
WHEN NOT MATCHED THEN INSERT VALUES (source.id, source.event_name, source.event_time);

Evolving the Schema

The following ALTER TABLE operations are supported:

ALTER TABLE my_catalog.sales.events ADD COLUMN source VARCHAR DEFAULT 'web';
ALTER TABLE my_catalog.sales.events DROP COLUMN source;
ALTER TABLE my_catalog.sales.events RENAME COLUMN id TO event_id;
ALTER TABLE my_catalog.sales.events ALTER COLUMN event_id TYPE BIGINT;
ALTER TABLE my_catalog.sales.events RENAME TO event_log;
ALTER TABLE my_catalog.sales.events ALTER COLUMN event_id SET DEFAULT 0;
ALTER TABLE my_catalog.sales.events ALTER COLUMN event_id DROP DEFAULT;

Copying Between DuckDB and Iceberg

Because the full DDL and DML set is supported, COPY FROM DATABASE can perform deep copies between Iceberg and DuckDB storage in either direction:

COPY FROM DATABASE duckdb_db TO my_catalog;
COPY FROM DATABASE my_catalog TO duckdb_db;

To copy an Iceberg catalog into DuckLake, see Interoperability with DuckLake.

Limitations

  • UPDATE and DELETE write positional deletes only; copy-on-write is not supported.
  • UPDATE and DELETE only support merge-on-read semantics. If a table sets write.update.mode or write.delete.mode to anything other than merge-on-read, the operation fails.
  • The write.target-file-size-bytes and write.parquet.row-group-size-bytes table properties are not honored for partitioned tables (see Partitioning).
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