Examples

Write a table to a Hive partitioned data set of Parquet files:

COPY orders TO 'orders'
(FORMAT parquet, PARTITION_BY (year, month));

Write a table to a Hive partitioned data set of CSV files, allowing overwrites:

COPY orders TO 'orders'
(FORMAT csv, PARTITION_BY (year, month), OVERWRITE_OR_IGNORE);

Write a table to a Hive partitioned data set of GZIP-compressed CSV files, setting explicit data files' extension:

COPY orders TO 'orders'
(FORMAT csv, PARTITION_BY (year, month), COMPRESSION gzip, FILE_EXTENSION 'csv.gz');

Partitioned Writes

When the PARTITION_BY clause is specified for the COPY statement, the files are written in a Hive partitioned folder hierarchy. The target is the name of the root directory (in the example above: orders). The files are written in-order in the file hierarchy. Currently, one file is written per thread to each directory.

orders
├── year=2021
│    ├── month=1
│    │   ├── data_1.parquet
│    │   └── data_2.parquet
│    └── month=2
│        └── data_1.parquet
└── year=2022
     ├── month=11
     │   ├── data_1.parquet
     │   └── data_2.parquet
     └── month=12
         └── data_1.parquet

The values of the partitions are automatically extracted from the data. Note that it can be very expensive to write a larger number of partitions as many files will be created. The ideal partition count depends on how large your data set is.

To limit the maximum number of files the system can keep open before flushing to disk when writing using PARTITION_BY, use the partitioned_write_max_open_files configuration option (default: 100):

SET partitioned_write_max_open_files = 10;

Best Practice

Writing data into many small partitions is expensive. It is generally recommended to have at least 100 MB of data per partition.

Filename Pattern

By default, files will be named data_0.parquet or data_0.csv. With the flag FILENAME_PATTERN a pattern with {i} or {uuid} can be defined to create specific filenames:

• {i} will be replaced by an index
• {uuid} will be replaced by a 128 bits long UUID

Write a table to a Hive partitioned data set of .parquet files, with an index in the filename:

COPY orders TO 'orders'
(FORMAT parquet, PARTITION_BY (year, month), OVERWRITE_OR_IGNORE, FILENAME_PATTERN 'orders_{i}');

Write a table to a Hive partitioned data set of .parquet files, with unique filenames:

COPY orders TO 'orders'
(FORMAT parquet, PARTITION_BY (year, month), OVERWRITE_OR_IGNORE, FILENAME_PATTERN 'file_{uuid}');

Overwriting

By default the partitioned write will not allow overwriting existing directories. On a local file system, the OVERWRITE and OVERWRITE_OR_IGNORE options remove the existing directories. On remote file systems, overwriting is not supported.

Appending

To append to an existing Hive partitioned directory structure, use the APPEND option:

COPY orders TO 'orders'
(FORMAT PARQUET, PARTITION_BY (year, month), APPEND);

Using the APPEND option result in a behavior similar the OVERWRITE_OR_IGNORE, FILENAME_PATTERN '{uuid}' options, but DuckDB performs an extra check for whether the file already exists and then regenerates the UUID in the rare event that it does (to avoid clashes).

Handling Slashes in Columns

To handle slashes in column names, use Percent-Encoding implemented by the url_encode function.