Reading and Writing Text Data in an Object Store
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The PXF object store connectors support plain delimited and comma-separated value format text data. This section describes how to use PXF to access text data in an object store, including how to create, query, and insert data into an external table that references files in the object store.
Note: Accessing text data from an object store is very similar to accessing text 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
Use the <objstore>:text
profile when you read plain text delimited or .csv data from an object store where each row is a single record. PXF supports the following <objstore>
profile prefixes:
Object Store | Profile Prefix |
---|---|
Azure Blob Storage | wasbs |
Azure Data Lake | adl |
Google Cloud Storage | gs |
Minio | s3 |
S3 | s3 |
The following syntax creates a Greenplum Database readable external table that references a simple 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>:text&SERVER=<server_name>[&<custom-option>=<value>[...]]')
FORMAT '[TEXT|CSV]' (delimiter[=|<space>][E]'<delim_value>');
The specific keywords and values used in the CREATE EXTERNAL TABLE command are described in the table below.
Keyword | Value |
---|---|
<path‑to‑file> | The absolute path to the directory or file in the S3 object store. |
PROFILE=<objstore>:text | The PROFILE keyword must identify the specific object store. For example, s3:text . |
SERVER=<server_name> | The named server configuration that PXF uses to access the data. |
FORMAT | Use FORMAT 'TEXT' when <path-to-file> references plain text delimited data.Use FORMAT 'CSV' when <path-to-file> references comma-separated value data. |
delimiter | The delimiter character in the data. For FORMAT 'CSV' , the default <delim_value> is a comma , . Preface the <delim_value> with an E when the value is an escape sequence. Examples: (delimiter=E'\t') , (delimiter ':') . |
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.If you are reading CSV-format data from S3, you can direct PXF to use the S3 Select Amazon service to retrieve the data. Refer to Using the Amazon S3 Select Service for more information about the PXF custom option used for this purpose.
Example: Reading Text Data from S3
Perform the following procedure to create a sample text file, copy the file to S3, and use the s3:text
profile to create two PXF external tables 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
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
Locally create a delimited plain text data file named
pxf_s3_simple.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_simple.txt
Note the use of the comma
,
to separate the four data fields.Copy the data file to the S3 directory you created in Step 1:
$ aws s3 cp /tmp/pxf_s3_simple.txt s3://BUCKET/pxf_examples/
Verify that the file now resides in S3:
$ aws s3 ls s3://BUCKET/pxf_examples/pxf_s3_simple.txt
Start the
psql
subsystem:$ psql -d postgres
Use the PXF
s3:text
profile to create a Greenplum Database external table that references thepxf_s3_simple.txt
file that you just created and added to S3. For example, if your server name iss3srvcfg
:postgres=# CREATE EXTERNAL TABLE pxf_s3_textsimple(location text, month text, num_orders int, total_sales float8) LOCATION ('pxf://BUCKET/pxf_examples/pxf_s3_simple.txt?PROFILE=s3:text&SERVER=s3srvcfg') FORMAT 'TEXT' (delimiter=E',');
Query the external table:
postgres=# SELECT * FROM pxf_s3_textsimple;
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)
Create a second external table that references
pxf_s3_simple.txt
, this time specifying theCSV
FORMAT
:postgres=# CREATE EXTERNAL TABLE pxf_s3_textsimple_csv(location text, month text, num_orders int, total_sales float8) LOCATION ('pxf://BUCKET/pxf_examples/pxf_s3_simple.txt?PROFILE=s3:text&SERVER=s3srvcfg') FORMAT 'CSV'; postgres=# SELECT * FROM pxf_s3_textsimple_csv;
When you specify
FORMAT 'CSV'
for comma-separated value data, nodelimiter
formatter option is required because comma is the default delimiter value.
Reading Text Data with Quoted Linefeeds
Use the <objstore>:text:multi
profile to read plain text data with delimited single- or multi- line records that include embedded (quoted) linefeed characters. The following syntax creates a Greenplum Database 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>:text:multi&SERVER=<server_name>[&<custom-option>=<value>[...]]')
FORMAT '[TEXT|CSV]' (delimiter[=|<space>][E]'<delim_value>');
The specific keywords and values used in the CREATE EXTERNAL TABLE command are described in the table below.
Keyword | Value |
---|---|
<path‑to‑file> | The absolute path to the directory or file in the S3 data store. |
PROFILE=<objstore>:text:multi | The PROFILE keyword must identify the specific object store. For example, s3:text:multi . |
SERVER=<server_name> | The named server configuration that PXF uses to access the data. |
FORMAT | Use FORMAT 'TEXT' when <path-to-file> references plain text delimited data.Use FORMAT 'CSV' when <path-to-file> references comma-separated value data. |
delimiter | The delimiter character in the data. For FORMAT 'CSV' , the default <delim_value> is a comma , . Preface the <delim_value> with an E when the value is an escape sequence. Examples: (delimiter=E'\t') , (delimiter ':') . |
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.
Example: Reading Multi-Line Text Data from S3
Perform the following steps to create a sample text file, copy the file to S3, and use the PXF s3:text:multi
profile to create a Greenplum Database readable 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
Create a second delimited plain text file:
$ vi /tmp/pxf_s3_multi.txt
Copy/paste the following data into
pxf_s3_multi.txt
:"4627 Star Rd. San Francisco, CA 94107":Sept:2017 "113 Moon St. San Diego, CA 92093":Jan:2018 "51 Belt Ct. Denver, CO 90123":Dec:2016 "93114 Radial Rd. Chicago, IL 60605":Jul:2017 "7301 Brookview Ave. Columbus, OH 43213":Dec:2018
Notice the use of the colon
:
to separate the three fields. Also notice the quotes around the first (address) field. This field includes an embedded line feed separating the street address from the city and state.Copy the text file to S3:
$ aws s3 cp /tmp/pxf_s3_multi.txt s3://BUCKET/pxf_examples/
Use the
s3:text:multi
profile to create an external table that references thepxf_s3_multi.txt
S3 file, making sure to identify the:
(colon) as the field separator. For example, if your server name iss3srvcfg
:postgres=# CREATE EXTERNAL TABLE pxf_s3_textmulti(address text, month text, year int) LOCATION ('pxf://BUCKET/pxf_examples/pxf_s3_multi.txt?PROFILE=s3:text:multi&SERVER=s3srvcfg') FORMAT 'CSV' (delimiter ':');
Notice the alternate syntax for specifying the
delimiter
.Query the
pxf_s3_textmulti
table:postgres=# SELECT * FROM pxf_s3_textmulti;
address | month | year --------------------------+-------+------ 4627 Star Rd. | Sept | 2017 San Francisco, CA 94107 113 Moon St. | Jan | 2018 San Diego, CA 92093 51 Belt Ct. | Dec | 2016 Denver, CO 90123 93114 Radial Rd. | Jul | 2017 Chicago, IL 60605 7301 Brookview Ave. | Dec | 2018 Columbus, OH 43213 (5 rows)
Writing Text Data
The “<objstore>:text” profiles support writing single line plain text data 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 Greenplum Database 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>:text&SERVER=<server_name>[&<custom-option>=<value>[...]]')
FORMAT '[TEXT|CSV]' (delimiter[=|<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‑dir> | The absolute path to the directory in the S3 data store. |
PROFILE=<objstore>:text | The PROFILE keyword must identify the specific object store. For example, s3:text . |
SERVER=<server_name> | The named server configuration that PXF uses to access the data. |
<custom‑option>=<value> | <custom-option>s are described below. |
FORMAT | Use FORMAT 'TEXT' to write plain, delimited text to <path-to-dir>.Use FORMAT 'CSV' to write comma-separated value text to <path-to-dir>. |
delimiter | The delimiter character in the data. For FORMAT 'CSV' , the default <delim_value> is a comma , . Preface the <delim_value> with an E when the value is an escape sequence. Examples: (delimiter=E'\t') , (delimiter ':') . |
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 an <objstore>:text
profile can optionally use record or block compression. The PXF <objstore>:text
profiles support the following compression codecs:
- org.apache.hadoop.io.compress.DefaultCodec
- org.apache.hadoop.io.compress.GzipCodec
- org.apache.hadoop.io.compress.BZip2Codec
You specify the compression codec via custom options in the CREATE EXTERNAL TABLE
LOCATION
clause. The <objstore>:text
profile support the following custom write options:
Option | Value Description |
---|---|
COMPRESSION_CODEC | The compression codec Java class name. If this option is not provided, Greenplum Database performs no data compression. Supported compression codecs include:org.apache.hadoop.io.compress.DefaultCodec org.apache.hadoop.io.compress.BZip2Codec org.apache.hadoop.io.compress.GzipCodec |
COMPRESSION_TYPE | The compression type to employ; supported values are RECORD (the default) or BLOCK . |
THREAD-SAFE | Boolean value determining if a table query can run in multi-threaded mode. The default value is TRUE . Set this option to FALSE to handle all requests in a single thread for operations that are not thread-safe (for example, compression). |
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.
Example: Writing Text Data to S3
This example utilizes the data schema introduced in Example: Reading Text Data from S3.
Column Name | Data Type |
---|---|
location | text |
month | text |
number_of_orders | int |
total_sales | float8 |
This example also optionally uses the Greenplum Database external table named pxf_s3_textsimple
that you created in that exercise.
Procedure
Perform the following procedure to create Greenplum Database writable external tables utilizing the same data schema as described above, one of which will employ compression. You will use the PXF s3:text
profile to write data to S3. You will also create a separate, readable external table to read the data that you wrote to S3.
Create a Greenplum Database writable external table utilizing the data schema described above. Write to the S3 directory
BUCKET/pxf_examples/pxfwrite_s3_textsimple1
. Create the table specifying a comma,
as the delimiter. For example, if your server name iss3srvcfg
:postgres=# CREATE WRITABLE EXTERNAL TABLE pxf_s3_writetbl_1(location text, month text, num_orders int, total_sales float8) LOCATION ('pxf://BUCKET/pxf_examples/pxfwrite_s3_textsimple1?PROFILE=s3:text&SERVER=s3srvcfg') FORMAT 'TEXT' (delimiter=',');
You specify the
FORMAT
subclausedelimiter
value as the single ascii comma character,
.Write a few individual records to the
pxfwrite_s3_textsimple1
S3 directory by invoking the SQLINSERT
command onpxf_s3_writetbl_1
:postgres=# INSERT INTO pxf_s3_writetbl_1 VALUES ( 'Frankfurt', 'Mar', 777, 3956.98 ); postgres=# INSERT INTO pxf_s3_writetbl_1 VALUES ( 'Cleveland', 'Oct', 3812, 96645.37 );
(Optional) Insert the data from the
pxf_s3_textsimple
table that you created in Example: Reading Text Data from S3 intopxf_s3_writetbl_1
:postgres=# INSERT INTO pxf_s3_writetbl_1 SELECT * FROM pxf_s3_textsimple;
Greenplum Database 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 Greenplum Database table that references the S3 directory:
postgres=# CREATE EXTERNAL TABLE pxf_s3_textsimple_r1(location text, month text, num_orders int, total_sales float8) LOCATION ('pxf://BUCKET/pxf_examples/pxfwrite_s3_textsimple1?PROFILE=s3:text&SERVER=s3srvcfg') FORMAT 'CSV';
You specify the
'CSV'
FORMAT
when you create the readable external table because you created the writable table with a comma,
as the delimiter character, the default delimiter for'CSV'
FORMAT
.Query the readable external table:
postgres=# SELECT * FROM pxf_s3_textsimple_r1 ORDER BY total_sales;
location | month | num_orders | total_sales -----------+-------+------------+------------- Rome | Mar | 87 | 1557.39 Frankfurt | Mar | 777 | 3956.98 Prague | Jan | 101 | 4875.33 Bangalore | May | 317 | 8936.99 Beijing | Jul | 411 | 11600.67 Cleveland | Oct | 3812 | 96645.37 (6 rows)
The
pxf_s3_textsimple_r1
table includes the records you individually inserted, as well as the full contents of thepxf_s3_textsimple
table if you performed the optional step.Create a second Greenplum Database writable external table, this time using Gzip compression and employing a colon
:
as the delimiter:postgres=# CREATE WRITABLE EXTERNAL TABLE pxf_s3_writetbl_2 (location text, month text, num_orders int, total_sales float8) LOCATION ('pxf://BUCKET/pxf_examples/pxfwrite_s3_textsimple2?PROFILE=s3:text&SERVER=s3srvcfg&COMPRESSION_CODEC=org.apache.hadoop.io.compress.GzipCodec') FORMAT 'TEXT' (delimiter=':');
Write a few records to the
pxfwrite_s3_textsimple2
S3 directory by inserting directly into thepxf_s3_writetbl_2
table:gpadmin=# INSERT INTO pxf_s3_writetbl_2 VALUES ( 'Frankfurt', 'Mar', 777, 3956.98 ); gpadmin=# INSERT INTO pxf_s3_writetbl_2 VALUES ( 'Cleveland', 'Oct', 3812, 96645.37 );
To query data from the newly-created S3 directory named
pxfwrite_s3_textsimple2
, you can create a readable external Greenplum Database table as described above that references this S3 directory and specifiesFORMAT 'CSV' (delimiter=':')
.