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Data Ingestion

CSV Files

CSV files can be read using the read_csv function, called either from within Python or directly from within SQL. By default, the read_csv function attempts to auto-detect the CSV settings by sampling from the provided file.

import duckdb
# read from a file using fully auto-detected settings
duckdb.read_csv("example.csv")
# read multiple CSV files from a folder
duckdb.read_csv("folder/*.csv")
# specify options on how the CSV is formatted internally
duckdb.read_csv("example.csv", header = False, sep = ",")
# override types of the first two columns
duckdb.read_csv("example.csv", dtype = ["int", "varchar"])
# use the (experimental) parallel CSV reader
duckdb.read_csv("example.csv", parallel = True)
# directly read a CSV file from within SQL
duckdb.sql("SELECT * FROM 'example.csv'")
# call read_csv from within SQL
duckdb.sql("SELECT * FROM read_csv('example.csv')")

See the CSV Import page for more information.

Parquet Files

Parquet files can be read using the read_parquet function, called either from within Python or directly from within SQL.

import duckdb
# read from a single Parquet file
duckdb.read_parquet("example.parquet")
# read multiple Parquet files from a folder
duckdb.read_parquet("folder/*.parquet")
# read a Parquet over https
duckdb.read_parquet("https://some.url/some_file.parquet")
# read a list of Parquet files
duckdb.read_parquet(["file1.parquet", "file2.parquet", "file3.parquet"])
# directly read a Parquet file from within SQL
duckdb.sql("SELECT * FROM 'example.parquet'")
# call read_parquet from within SQL
duckdb.sql("SELECT * FROM read_parquet('example.parquet')")

See the Parquet Loading page for more information.

JSON Files

JSON files can be read using the read_json function, called either from within Python or directly from within SQL. By default, the read_json function will automatically detect if a file contains newline-delimited JSON or regular JSON, and will detect the schema of the objects stored within the JSON file.

import duckdb
# read from a single JSON file
duckdb.read_json("example.json")
# read multiple JSON files from a folder
duckdb.read_json("folder/*.json")
# directly read a JSON file from within SQL
duckdb.sql("SELECT * FROM 'example.json'")
# call read_json from within SQL
duckdb.sql("SELECT * FROM read_json_auto('example.json')")

DataFrames & Arrow Tables

DuckDB is automatically able to query a Pandas DataFrame, Polars DataFrame, or Arrow object that is stored in a Python variable by name. Accessing these is made possible by replacement scans.

DuckDB supports querying multiple types of Apache Arrow objects including tables, datasets, RecordBatchReaders, and scanners. See the Python guides for more examples.

import duckdb
import pandas as pd
test_df = pd.DataFrame.from_dict({"i": [1, 2, 3, 4], "j": ["one", "two", "three", "four"]})
duckdb.sql("SELECT * FROM test_df").fetchall()
# [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]

DuckDB also supports “registering” a DataFrame or Arrow object as a virtual table, comparable to a SQL VIEW. This is useful when querying a DataFrame/Arrow object that is stored in another way (as a class variable, or a value in a dictionary). Below is a Pandas example:

If your Pandas DataFrame is stored in another location, here is an example of manually registering it:

import duckdb
import pandas as pd
my_dictionary = {}
my_dictionary["test_df"] = pd.DataFrame.from_dict({"i": [1, 2, 3, 4], "j": ["one", "two", "three", "four"]})
duckdb.register("test_df_view", my_dictionary["test_df"])
duckdb.sql("SELECT * FROM test_df_view").fetchall()
# [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]

You can also create a persistent table in DuckDB from the contents of the DataFrame (or the view):

# create a new table from the contents of a DataFrame
con.execute("CREATE TABLE test_df_table AS SELECT * FROM test_df")
# insert into an existing table from the contents of a DataFrame
con.execute("INSERT INTO test_df_table SELECT * FROM test_df")

Pandas DataFrames – object Columns

pandas.DataFrame columns of an object dtype require some special care, since this stores values of arbitrary type. To convert these columns to DuckDB, we first go through an analyze phase before converting the values. In this analyze phase a sample of all the rows of the column are analyzed to determine the target type. This sample size is by default set to 1000. If the type picked during the analyze step is incorrect, this will result in a “Failed to cast value:” error, in which case you will need to increase the sample size. The sample size can be changed by setting the pandas_analyze_sample config option.

# example setting the sample size to 100k
duckdb.default_connection.execute("SET GLOBAL pandas_analyze_sample = 100_000")

Object Conversion

This is a mapping of Python object types to DuckDB Logical Types:

  • None -> NULL
  • bool -> BOOLEAN
  • datetime.timedelta -> INTERVAL
  • str -> VARCHAR
  • bytearray -> BLOB
  • memoryview -> BLOB
  • decimal.Decimal -> DECIMAL / DOUBLE
  • uuid.UUID -> UUID

The rest of the conversion rules are as follows.

int

Since integers can be of arbitrary size in Python, there is not a one-to-one conversion possible for ints. Intead we perform these casts in order until one succeeds:

  • BIGINT
  • INTEGER
  • UBIGINT
  • UINTEGER
  • DOUBLE

When using the DuckDB Value class, it’s possible to set a target type, which will influence the conversion.

float

These casts are tried in order until one succeeds:

  • DOUBLE
  • FLOAT

datetime.datetime

For datetime we will check pandas.isnull if it’s available and return NULL if it returns true. We check against datetime.datetime.min and datetime.datetime.max to convert to -inf and +inf respectively.

If the datetime has tzinfo, we will use TIMESTAMPTZ, otherwise it becomes TIMESTAMP.

datetime.time

If the time has tzinfo, we will use TIMETZ, otherwise it becomes TIME.

datetime.date

date converts to the DATE type. We check against datetime.date.min and datetime.date.max to convert to -inf and +inf respectively.

bytes

bytes converts to BLOB by default, when it’s used to construct a Value object of type BITSTRING, it maps to BITSTRING instead.

list

list becomes a LIST type of the “most permissive” type of its children, for example:

my_list_value = [
    12345,
    "test"
]

Will become VARCHAR[] because 12345 can convert to VARCHAR but test can not convert to INTEGER.

[12345, test]

dict

The dict object can convert to either STRUCT(...) or MAP(..., ...) depending on its structure. If the dict has a structure similar to:

my_map_dict = {
    "key": [
        1, 2, 3
    ],
    "value": [
        "one", "two", "three"
    ]
}

Then we’ll convert it to a MAP of key-value pairs of the two lists zipped together. The example above becomes a MAP(INTEGER, VARCHAR):

{1=one, 2=two, 3=three}

The names of the fields matter and the two lists need to have the same size.

Otherwise we’ll try to convert it to a STRUCT.

my_struct_dict = {
    1: "one",
    "2": 2,
    "three": [1, 2, 3],
    False: True
}

Becomes:

{'1': one, '2': 2, 'three': [1, 2, 3], 'False': true}

Every key of the dictionary is converted to string.

tuple

tuple converts to LIST by default, when it’s used to construct a Value object of type STRUCT it will convert to STRUCT instead.

numpy.ndarray and numpy.datetime64

ndarray and datetime64 are converted by calling tolist() and converting the result of that.

About this page

Last modified: 2024-02-24