Reading and Writing Fixed-Width Text Data in an Object Store

The PXF object store connectors support reading and writing fixed-width text using the SynxDB 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 object store.

Note: Accessing fixed-width text data from an object store is very similar to accessing such data in HDFS.

Prerequisites

Ensure that you have met the PXF Object Store Prerequisites before you attempt to read data from or write data to an object store.

Reading Text Data with Fixed Widths

Use the <objstore>:fixedwidth profile when you read fixed-width text from an object store where each line is a single record. PXF supports the following <objstore> profile prefixes:

The following syntax creates a SynxDB readable external table that references such a text file in an object store: 

CREATE EXTERNAL TABLE <table_name> 
    ( <column_name> <data_type> [, ...] | LIKE <other_table> )
LOCATION ('pxf://<path-to-file>?PROFILE=<objstore>: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 SynxDB CREATE EXTERNAL TABLE command are described in the table below.

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

If you are accessing an S3 object store, you can provide S3 credentials via custom options in the CREATE EXTERNAL TABLE command as described in Overriding the S3 Server Configuration with DDL.

About Specifying field_name and width

SynxDB 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 SynxDB fixed width custom formatter documentation for more information about the formatter options.

About the line_delim and NEWLINE Formatter Options

By default, SynxDB 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 SynxDB fixed width custom formatter documentation for more information about the formatter options.

Example: Reading Fixed-Width Text Data on S3

Perform the following procedure to create a sample text file, copy the file to S3, and use the s3:fixedwidth profile to create a PXF external table to query the data.

To run this example, you must:

• Have the AWS CLI tools installed on your system
• Know your AWS access ID and secret key
• Have write permission to an S3 bucket

Procedure:

1. Create a directory in S3 for PXF example data files. For example, if you have write access to an S3 bucket named BUCKET:

   $ aws s3 mb s3://BUCKET/pxf_examples
   

2. Locally create a plain text data file named pxf_s3_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_s3_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_s3_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 the S3 directory that you created in Step 1:

   $ aws s3 cp /tmp/pxf_s3_fixedwidth.txt s3://BUCKET/pxf_examples/
   

4. Verify that the file now resides in S3:

   $ aws s3 ls s3://BUCKET/pxf_examples/pxf_s3s_fixedwidth.txt
   

5. Start the psql subsystem:

   $ psql -d postgres
   

6. Use the PXF s3:fixedwidth profile to create a SynxDB external table that references the pxf_s3_fixedwidth.txt file that you just created and added to S3. For example, if your server name is s3srvcfg:

   postgres=# CREATE EXTERNAL TABLE pxf_s3_fixedwidth_r(location text, month text, num_orders int, total_sales float8)
                LOCATION ('pxf://data/pxf_examples/pxf_s3_fixedwidth.txt?PROFILE=s3:fixedwidth&SERVER=s3srvcfg&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_s3_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

The <objstore>:fixedwidth profiles support writing fixed-width text to an object store. When you create a writable external table with PXF, you specify the name of a directory. 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 directory.

Use the following syntax to create a SynxDB writable external table that references an object store directory: 

CREATE WRITABLE EXTERNAL TABLE <table_name> 
    ( <column_name> <data_type> [, ...] | LIKE <other_table> )
LOCATION ('pxf://<path-to-dir>
    ?PROFILE=<objstore>: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.

Writable external tables that you create using the <objstore>: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:

Example: Writing Fixed-Width Text Data to S3

This example utilizes the data schema introduced in Example: Reading Fixed-Width Text Data on S3.

| 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 SynxDB writable external table utilizing the same data schema as described above. You will use the PXF s3:fixedwidth profile to write data to S3. You will also create a separate, readable external table to read the data that you wrote to #3.

1. Create a SynxDB writable external table utilizing the data schema described above. Write to the S3 directory BUCKET/pxf_examples/fixedwidth_write. Create the table specifying \n as the line delimiter. For example, if your server name is s3srvcfg:

   postgres=# CREATE WRITABLE EXTERNAL TABLE pxf_s3_fixedwidth_w(location text, month text, num_orders int, total_sales float8)
                LOCATION ('pxf://BUCKET/pxf_examples/fixedwidth_write?PROFILE=s3:fixedwidth&SERVER=s3srvcfg')
              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 S3 directory by using the INSERT command on the pxf_s3_fixedwidth_w table:

   postgres=# INSERT INTO pxf_s3_fixedwidth_w VALUES ( 'Frankfurt', 'Mar', 777, 3956.98 );
   postgres=# INSERT INTO pxf_s3_fixedwidth_w VALUES ( 'Cleveland', 'Oct', 3812, 96645.37 );
   

3. SynxDB does not support directly querying a writable external table. To query the data that you just added to S3, you must create a readable external SynxDB table that references the S3 directory:

   postgres=# CREATE EXTERNAL TABLE pxf_s3_fixedwidth_r2(location text, month text, num_orders int, total_sales float8)
                LOCATION ('pxf://BUCKET/pxf_examples/fixedwidth_write?PROFILE=s3:fixedwidth&SERVER=s3srvcfg')
              FORMAT 'CUSTOM' (formatter='fixedwidth_in', location='15', month='4', num_orders='6', total_sales='10');
   

4. Query the readable external table:

   postgres=# SELECT * FROM pxf_s3_fixedwidth_r2 ORDER BY total_sales;
   

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