Reading and Writing Fixed-Width Text Data

The PXF HDFS Connector supports reading and writing fixed-width text using the Greenplum Database fixed width custom formatter. This section describes how to use PXF to access fixed-width text, including how to create, query, and insert data into an external table that references files in the HDFS data store.

PXF supports reading or writing fixed-width text that is compressed with the default, bzip2, and gzip codecs.

Prerequisites

Ensure that you have met the PXF Hadoop Prerequisites before you attempt to read fixed-width text from, or write fixed-width text to, HDFS.

Reading Text Data with Fixed Widths

Use the hdfs:fixedwidth profile when you read fixed-width text where each line is considered a single record. The following syntax creates a Greenplum Database readable external table that references such a text file on HDFS: 

CREATE EXTERNAL TABLE <table_name> 
    ( <column_name> <data_type> [, ...] | LIKE <other_table> )
LOCATION ('pxf://<path-to-hdfs-file>?PROFILE=hdfs:fixedwidth[&SERVER=<server_name>][&NEWLINE=<bytecode>][&IGNORE_MISSING_PATH=<boolean>]')
FORMAT 'CUSTOM' (FORMATTER='fixedwidth_in', <field_name>='<width>' [, ...] [, line_delim[=|<space>][E]'<delim_value>']);

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

Keyword	Value
<path‑to‑hdfs‑file>	The path to the directory or file in the HDFS data store. When the <server_name> configuration includes a pxf.fs.basePath property setting, PXF considers <path‑to‑hdfs‑file> to be relative to the base path specified. Otherwise, PXF considers it to be an absolute path. <path‑to‑hdfs‑file> must not specify a relative path nor include the dollar sign ($) character.
PROFILE	Use PROFILE=hdfs:fixedwidth when <path-to-hdfs-file> references fixed-width text data.
SERVER=<server_name>	The named server configuration that PXF uses to access the data. PXF uses the default server if not specified.
NEWLINE=<bytecode>	When the line_delim formatter option contains \r, \r\n, or a set of custom escape characters, you must set <bytecode> to CR, CRLF, or the set of bytecode characters, respectively.
IGNORE_MISSING_PATH=<boolean>	Specify 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 dataset.
FORMAT ‘CUSTOM’	Use FORMAT ‘CUSTOM’ with FORMATTER='fixedwidth_in' (read).
<field_name>=‘<width>’	The name and the width of the field. For example: first_name='15' specifies that the first_name field is 15 characters long. By default, when the field value is less than <width> size, Greenplum Database expects the field to be right-padded with spaces to that size.
line_delim	The line delimiter character in the data. Preface the <delim_value> with an E when the value is an escape sequence. Examples: line_delim=E'\n', line_delim 'aaa'. The default value is '\n'.

Note: PXF does not support the (HEADER) formatter option in the CREATE EXTERNAL TABLE command.

About Specifying field_name and width

Greenplum Database loads all fields in a line of fixed-width data in their physical order. The <field_name>s that you specify in the FORMAT options must match the order that you define the columns in the CREATE [WRITABLE] EXTERNAL TABLE command. You specify the size of each field in the <width> value.

Refer to the Greenplum Database fixed width custom formatter documentation for more information about the formatter options.

About the line_delim and NEWLINE Formatter Options

By default, Greenplum Database uses the \n (LF) character for the new line delimiter. When the line delimiter for the external file is also \n, you need not specify the line_delim option. If the line_delim formatter option is provided and contains \r (CR), \r\n (CRLF), or a set of custom escape characters, you must specify the NEWLINE option in the external table LOCATION clause, and set the value to CR, CRLF or the set of bytecode characters, respectively.

Refer to the Greenplum Database fixed width custom formatter documentation for more information about the formatter options.

Example: Reading Fixed-Width Text Data on HDFS

Perform the following procedure to create a sample text file, copy the file to HDFS, and use the hdfs:fixedwidth profile and the default PXF server to create a PXF external table to query the data:

1. Create an HDFS directory for PXF example data files. For example:

   $ hdfs dfs -mkdir -p /data/pxf_examples
   

2. Create a plain text data file named pxf_hdfs_fixedwidth.txt:

   $ echo 'Prague         Jan 101   4875.33   
   Rome           Mar 87    1557.39   
   Bangalore      May 317   8936.99   
   Beijing        Jul 411   11600.67  ' > /tmp/pxf_hdfs_fixedwidth.txt
   

   In this sample file, the first field is 15 characters long, the second is 4 characters, the third is 6 characters, and the last field is 10 characters long.

   Note

   Open the /tmp/pxf_hdfs_fixedwidth.txt file in the editor of your choice, and ensure that the last field is right-padded with spaces to 10 characters in size.

3. Copy the data file to HDFS:

   $ hdfs dfs -put /tmp/pxf_hdfs_fixedwidth.txt /data/pxf_examples/
   

4. Display the contents of the pxf_hdfs_fixedwidth.txt file stored in HDFS:

   $ hdfs dfs -cat /data/pxf_examples/pxf_hdfs_fixedwidth.txt
   

5. Start the psql subsystem:

   $ psql -d postgres
   

6. Use the PXF hdfs:fixedwidth profile to create a Greenplum Database external table that references the pxf_hdfs_fixedwidth.txt file that you just created and added to HDFS:

   postgres=# CREATE EXTERNAL TABLE pxf_hdfs_fixedwidth_r(location text, month text, num_orders int, total_sales float8)
                LOCATION ('pxf://data/pxf_examples/pxf_hdfs_fixedwidth.txt?PROFILE=hdfs:fixedwidth&NEWLINE=CRLF')
              FORMAT 'CUSTOM' (formatter='fixedwidth_in', location='15', month='4', num_orders='6', total_sales='10', line_delim=E'\r\n');
   

7. Query the external table:

   postgres=# SELECT * FROM pxf_hdfs_fixedwidth_r;
   

      location    | month | num_orders | total_sales 
   ---------------+-------+------------+-------------
    Prague        | Jan   |        101 |     4875.33
    Rome          | Mar   |         87 |     1557.39
    Bangalore     | May   |        317 |     8936.99
    Beijing       | Jul   |        411 |    11600.67
   (4 rows)
   

Writing Fixed-Width Text Data to HDFS

The PXF HDFS connector hdfs:fixedwidth profile supports writing fixed-width text to HDFS. When you create a writable external table with the PXF HDFS connector, you specify the name of a directory on HDFS. 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.

Note: External tables that you create with a writable profile can only be used for INSERT operations. If you want to query the data that you inserted, you must create a separate readable external table that references the HDFS directory.

Use the following syntax to create a Greenplum Database writable 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:fixedwidth[&SERVER=<server_name>][&NEWLINE=<bytecode>][&<write-option>=<value>[...]]')
FORMAT 'CUSTOM' (FORMATTER='fixedwidth_out' [, <field_name>='<width>'] [, ...] [, line_delim[=|<space>][E]'<delim_value>']);
[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‑dir>	The path to the directory in the HDFS data store. When the <server_name> configuration includes a pxf.fs.basePath property setting, PXF considers <path‑to‑hdfs‑dir> to be relative to the base path specified. Otherwise, PXF considers it to be an absolute path. <path‑to‑hdfs‑dir> must not specify a relative path nor include the dollar sign ($) character.
PROFILE	Use PROFILE=hdfs:fixedwidth to write fixed-width data to <path-to-hdfs-file>.
SERVER=<server_name>	The named server configuration that PXF uses to access the data. PXF uses the default server if not specified.
NEWLINE=<bytecode>	When the line_delim formatter option contains \r, \r\n, or a set of custom escape characters, you must set <bytecode> to CR, CRLF or the set of bytecode characters, respectively.
<write‑option>=<value>	<write-option>s are described below.
FORMAT ‘CUSTOM’	Use FORMAT ‘CUSTOM’ with FORMATTER='fixedwidth_out' (write).
<field_name>=‘<width>’	The name and the width of the field. For example: first_name='15' specifies that the first_name field is 15 characters long. By default, when writing to the external file and the field value is less than <width> size, Greenplum Database right-pads the field with spaces to <width> size.
line_delim	The line delimiter character in the data. Preface the <delim_value> with an E when the value is an escape sequence. Examples: line_delim=E'\n', line_delim 'aaa'. The default value is '\n'.
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.

Writable external tables that you create using the hdfs:fixedwidth profile can optionally use record or block compression. You specify the compression codec via an option in the CREATE WRITABLE EXTERNAL TABLE LOCATION clause:

Write Option	Value Description
COMPRESSION_CODEC	The compression codec alias. Supported compression codecs for writing fixed-width text data include: default, bzip2, gzip, and uncompressed. If this option is not provided, Greenplum Database performs no data compression.

Example: Writing Fixed-Width Text Data to HDFS

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

Column Name	Width	Data Type
location	15	text
month	4	text
number_of_orders	6	int
total_sales	10	float8

Procedure

Perform the following procedure to create a Greenplum Database writable external table utilizing the same data schema as described above. You will use the PXF hdfs:fixedwidth profile and the default PXF server to write data to the underlying HDFS directory. You will also create a separate, readable external table to read the data that you wrote to the HDFS directory.

1. Create a Greenplum Database writable external table utilizing the data schema described above. Write to the HDFS directory /data/pxf_examples/fixedwidth_write. Create the table specifying \n as the line delimiter:

   postgres=# CREATE WRITABLE EXTERNAL TABLE pxf_hdfs_fixedwidth_w(location text, month text, num_orders int, total_sales float8)
                LOCATION ('pxf://data/pxf_examples/fixedwidth_write?PROFILE=hdfs:fixedwidth')
              FORMAT 'CUSTOM' (formatter='fixedwidth_out', location='15', month='4', num_orders='6', total_sales='10');
   

2. Write a few individual records to the fixedwidth_write HDFS directory by using the INSERT command on the pxf_hdfs_fixedwidth_w table:

   postgres=# INSERT INTO pxf_hdfs_fixedwidth_w VALUES ( 'Frankfurt', 'Mar', 777, 3956.98 );
   postgres=# INSERT INTO pxf_hdfs_fixedwidth_w VALUES ( 'Cleveland', 'Oct', 3812, 96645.37 );
   

3. In another terminal window, use the cat command on the fixedwidth_write directory to display the data that you just added to HDFS:

   $ hdfs dfs -cat /data/pxf_examples/fixedwidth_write/*
   Frankfurt      Mar 777   3956.98   
   Cleveland      Oct 3812  96645.37  
   

4. Greenplum Database does not support directly querying a writable external table. To query the data that you just added to HDFS, you must create a readable external Greenplum Database table that references the HDFS directory:

   postgres=# CREATE EXTERNAL TABLE pxf_hdfs_fixedwidth_r2(location text, month text, num_orders int, total_sales float8)
                LOCATION ('pxf://data/pxf_examples/fixedwidth_write?PROFILE=hdfs:fixedwidth')
              FORMAT 'CUSTOM' (formatter='fixedwidth_in', location='15', month='4', num_orders='6', total_sales='10');
   

5. Query the readable external table:

   postgres=# SELECT * FROM pxf_hdfs_fixedwidth_r2 ORDER BY total_sales;
   

    location  | month | num_orders | total_sales 
   -----------+-------+------------+-------------
    Frankfurt | Mar   |        777 |     3956.98
    Cleveland | Oct   |       3812 |    96645.37
   (2 rows)