Reading and Writing HDFS Parquet Data

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Use the PXF HDFS connector to read and write Parquet-format data. This section decribes how to read and write HDFS files that are stored in Parquet format, including how to create, query, and insert into external tables that reference files in the HDFS data store.

PXF currently supports reading and writing primitive Parquet data types only.

Prerequisites

Ensure that you have met the PXF Hadoop Prerequisites before you attempt to read data from or write data to HDFS.

Data Type Mapping

To read and write Parquet primitive data types in Greenplum Database, map Parquet data values to Greenplum Database columns of the same type.

Parquet supports a small set of primitive data types, and uses metadata annotations to extend the data types that it supports. These annotations specify how to interpret the primitive type. For example, Parquet stores both INTEGER and DATE types as the INT32 primitive type. An annotation identifies the original type as a DATE.

Read Mapping

PXF uses the following data type mapping when reading Parquet data:

Parquet Data Type Original Type PXF/Greenplum Data Type
binary (byte_array) Date Date
binary (byte_array) Timestamp_millis Timestamp
binary (byte_array) all others Text
binary (byte_array) Bytea
boolean Boolean
double Float8
fixed_len_byte_array Numeric
float Real
int32 Date Date
int32 Decimal Numeric
int32 int_8 Smallint
int32 int_16 Smallint
int32 Integer
int64 Decimal Numeric
int64 Bigint
int96 Timestamp

Note: PXF supports filter predicate pushdown on all parquet data types listed above, except the fixed_len_byte_array and int96 types.

Write Mapping

PXF uses the following data type mapping when writing Parquet data:

PXF/Greenplum Data Type Original Type Parquet Data Type
Boolean boolean
Bytea binary
Bigint int64
SmallInt int_16 int32
Integer int32
Real float
Float8 double
Numeric/Decimal Decimal fixed_len_byte_array
Timestamp1 int96
Timestamptz2 int96
Date utf8 binary
Time utf8 binary
Varchar utf8 binary
Text utf8 binary
OTHERS UNSUPPORTED


1 PXF localizes a Timestamp to the current system timezone and converts it to universal time (UTC) before finally converting to int96.
2 PXF converts a Timestamptz to a UTC timestamp and then converts to int96. PXF loses the time zone information during this conversion.

Creating the External Table

The PXF HDFS connector hdfs:parquet profile supports reading and writing HDFS data in Parquet-format. When you insert records into a writable external table, the block(s) of data that you insert are written to one or more files in the directory that you specified.

Use the following syntax to create a Greenplum Database external table that references an HDFS directory:

CREATE [WRITABLE] EXTERNAL TABLE <table_name>
    ( <column_name> <data_type> [, ...] | LIKE <other_table> )
LOCATION ('pxf://<path-to-hdfs-dir>
    ?PROFILE=hdfs:parquet[&SERVER=<server_name>][&<custom-option>=<value>[...]]')
FORMAT 'CUSTOM' (FORMATTER='pxfwritable_import'|'pxfwritable_export');
[DISTRIBUTED BY (<column_name> [, ... ] ) | DISTRIBUTED RANDOMLY];

The specific keywords and values used in the CREATE EXTERNAL TABLE command are described in the table below.

Keyword Value
<path‑to‑hdfs‑file> The absolute path to the directory in the HDFS data store.
PROFILE The PROFILE keyword must specify hdfs:parquet.
SERVER=<server_name> The named server configuration that PXF uses to access the data. Optional; PXF uses the default server if not specified.
<custom‑option> <custom-option>s are described below.
FORMAT ‘CUSTOM’ Use FORMATCUSTOM’ with (FORMATTER='pxfwritable_export') (write) or (FORMATTER='pxfwritable_import') (read).
DISTRIBUTED BY If you want to load data from an existing Greenplum Database table into the writable external table, consider specifying the same distribution policy or <column_name> on both tables. Doing so will avoid extra motion of data between segments on the load operation.

The PXF hdfs:parquet profile supports the following read option. You specify this option in the CREATE EXTERNAL TABLE LOCATION clause:

Read Option Value Description
IGNORE_MISSING_PATH A Boolean value that specifies the action to take when <path-to-hdfs-file> is missing or invalid. The default value is false, PXF returns an error in this situation. When the value is true, PXF ignores missing path errors and returns an empty fragment.

The PXF hdfs:parquet profile supports encoding- and compression-related write options. You specify these write options in the CREATE WRITABLE EXTERNAL TABLE LOCATION clause. The hdfs:parquet profile supports the following custom write options:

Write Option Value Description
COMPRESSION_CODEC The compression codec alias. Supported compression codecs for writing Parquet data include: snappy, gzip, lzo, and uncompressed . If this option is not provided, PXF compresses the data using snappy compression.
ROWGROUP_SIZE A Parquet file consists of one or more row groups, a logical partitioning of the data into rows. ROWGROUP_SIZE identifies the size (in bytes) of the row group. The default row group size is 8 * 1024 * 1024 bytes.
PAGE_SIZE A row group consists of column chunks that are divided up into pages. PAGE_SIZE is the size (in bytes) of such a page. The default page size is 1024 * 1024 bytes.
DICTIONARY_PAGE_SIZE Dictionary encoding is enabled by default when PXF writes Parquet files. There is a single dictionary page per column, per row group. DICTIONARY_PAGE_SIZE is similar to PAGE_SIZE, but for the dictionary. The default dictionary page size is 512 * 1024 bytes.
PARQUET_VERSION The Parquet version; values v1 and v2 are supported. The default Parquet version is v1.
SCHEMA The location of the Parquet schema file on the file system of the specified SERVER.

Note: You must explicitly specify uncompressed if you do not want PXF to compress the data.

Parquet files that you write to HDFS with PXF have the following naming format: <file>.<compress_extension>.parquet, for example 1547061635-0000004417_0.gz.parquet.

Example

This example utilizes the data schema introduced in Example: Reading Text Data on HDFS.

Column Name Data Type
location text
month text
number_of_orders int
total_sales float8

In this example, you create a Parquet-format writable external table that uses the default PXF server to reference Parquet-format data in HDFS, insert some data into the table, and then create a readable external table to read the data.

  1. Use the hdfs:parquet profile to create a writable external table. For example:

    postgres=# CREATE WRITABLE EXTERNAL TABLE pxf_tbl_parquet (location text, month text, number_of_orders int, total_sales double precision)
        LOCATION ('pxf://data/pxf_examples/pxf_parquet?PROFILE=hdfs:parquet')
      FORMAT 'CUSTOM' (FORMATTER='pxfwritable_export');
    
  2. Write a few records to the pxf_parquet HDFS directory by inserting directly into the pxf_tbl_parquet table. For example:

    postgres=# INSERT INTO pxf_tbl_parquet VALUES ( 'Frankfurt', 'Mar', 777, 3956.98 );
    postgres=# INSERT INTO pxf_tbl_parquet VALUES ( 'Cleveland', 'Oct', 3812, 96645.37 );
    
  3. Recall that Greenplum Database does not support directly querying a writable external table. To read the data in pxf_parquet, create a readable external Greenplum Database referencing this HDFS directory:

    postgres=# CREATE EXTERNAL TABLE read_pxf_parquet(location text, month text, number_of_orders int, total_sales double precision)
        LOCATION ('pxf://data/pxf_examples/pxf_parquet?PROFILE=hdfs:parquet')
        FORMAT 'CUSTOM' (FORMATTER='pxfwritable_import');
    
  4. Query the readable external table read_pxf_parquet:

    postgres=# SELECT * FROM read_pxf_parquet ORDER BY total_sales;
    
     location  | month | number_of_orders | total_sales
    -----------+-------+------------------+-------------
     Frankfurt | Mar   |              777 |     3956.98
     Cleveland | Oct   |             3812 |     96645.4
    (2 rows)