parquet_fdw

Read-only Apache Parquet foreign data wrapper for PostgreSQL.

Installation

parquet_fdw requires libarrow and libparquet installed in your system (requires version 0.15+, for previous versions use branch arrow-0.14). Please refer to libarrow installation page or building guide. To build parquet_fdw run:

make install

or in case when PostgreSQL is installed in a custom location:

make install PG_CONFIG=/path/to/pg_config

It is possible to pass additional compilation flags through either custom CCFLAGS or standard PG_CFLAGS, PG_CXXFLAGS, PG_CPPFLAGS variables.

After extension was successfully installed run in psql:

create extension parquet_fdw;

Basic usage

To start using parquet_fdw one should first create a server and user mapping. For example:

create server parquet_srv foreign data wrapper parquet_fdw;
create user mapping for postgres server parquet_srv options (user 'postgres');

Now you should be able to create foreign table for Parquet files.

create foreign table userdata (
    id           int,
    first_name   text,
    last_name    text
)
server parquet_srv
options (
    filename '/mnt/userdata1.parquet'
);

Advanced

Currently parquet_fdw supports the following column types:

Arrow type	SQL type
INT8	INT2
INT16	INT2
INT32	INT4
INT64	INT8
FLOAT	FLOAT4
DOUBLE	FLOAT8
TIMESTAMP	TIMESTAMP
DATE32	DATE
STRING	TEXT
BINARY	BYTEA
LIST	ARRAY
MAP	JSONB

Currently parquet_fdw doesn't support structs and nested lists.

Foreign table may be created for a single Parquet file and for a set of files. It is also possible to specify a user defined function, which would return a list of file paths. Depending on the number of files and table options parquet_fdw may use one of the following execution strategies:

Strategy	Description
Single File	Basic single file reader
Multifile	Reader which process Parquet files one by one in sequential manner
Multifile Merge	Reader which merges presorted Parquet files so that the produced result is also ordered; used when sorted option is specified and the query plan implies ordering (e.g. contains ORDER BY clause)
Caching Multifile Merge	Same as Multifile Merge, but keeps the number of simultaneously open files limited; used when the number of specified Parquet files exceeds max_open_files

Following table options are supported:

• filename - space separated list of paths to Parquet files to read;
• sorted - space separated list of columns that Parquet files are presorted by; that would help postgres to avoid redundant sorting when running query with ORDER BY clause or in other cases when having a presorted set is beneficial (Group Aggregate, Merge Join);
• files_in_order - specifies that files specified by filename or returned by files_func are ordered according to sorted option and have no intersection rangewise; this allows to use Gather Merge node on top of parallel Multifile scan (default false);
• use_mmap - whether memory map operations will be used instead of file read operations (default false);
• use_threads - enables Apache Arrow's parallel columns decoding/decompression (default false);
• files_func - user defined function that is used by parquet_fdw to retrieve the list of parquet files on each query; function must take one JSONB argument and return text array of full paths to parquet files;
• files_func_arg - argument for the function, specified by files_func;
• max_open_files - the limit for the number of Parquet files open simultaneously.

GUC variables:

• parquet_fdw.use_threads - global switch that allow user to enable or disable threads (default true);
• parquet_fdw.enable_multifile - enable Multifile reader (default true).
• parquet_fdw.enable_multifile_merge - enable Multifile Merge reader (default true).

Parallel queries

parquet_fdw also supports parallel query execution (not to confuse with multi-threaded decoding feature of Apache Arrow).

Import

parquet_fdw also supports IMPORT FOREIGN SCHEMA command to discover parquet files in the specified directory on filesystem and create foreign tables according to those files. It can be used as follows:

import foreign schema "/path/to/directory"
from server parquet_srv
into public;

It is important that remote_schema here is a path to a local filesystem directory and is double quoted.

Another way to import parquet files into foreign tables is to use import_parquet or import_parquet_explicit:

create function import_parquet(
    tablename   text,
    schemaname  text,
    servername  text,
    userfunc    regproc,
    args        jsonb,
    options     jsonb)

create function import_parquet_explicit(
    tablename   text,
    schemaname  text,
    servername  text,
    attnames    text[],
    atttypes    regtype[],
    userfunc    regproc,
    args        jsonb,
    options     jsonb)

The only difference between import_parquet and import_parquet_explicit is that the latter allows to specify a set of attributes (columns) to import. attnames and atttypes here are the attributes names and attributes types arrays respectively (see the example below).

userfunc is a user-defined function. It must take a jsonb argument and return a text array of filesystem paths to parquet files to be imported. args is user-specified jsonb object that is passed to userfunc as its argument. A simple implementation of such function and its usage may look like this:

create function list_parquet_files(args jsonb)
returns text[] as
$$
begin
    return array_agg(args->>'dir' || '/' || filename)
           from pg_ls_dir(args->>'dir') as files(filename)
           where filename ~~ '%.parquet';
end
$$
language plpgsql;

select import_parquet_explicit(
    'abc',
    'public',
    'parquet_srv',
    array['one', 'three', 'six'],
    array['int8', 'text', 'bool']::regtype[],
    'list_parquet_files',
    '{"dir": "/path/to/directory"}',
    '{"sorted": "one"}'
);