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CREATE EXTERNAL TABLE

Defines a new external table.

Synopsis

CREATE [READABLE] EXTERNAL [TEMPORARY | TEMP] TABLE <table_name>
    ( <column_name> <data_type> [, ...] | LIKE <other_table>)
     LOCATION ('file://<seghost>[:<port>]/<path>/<file>' [, ...])
       | ('gpfdist://<filehost>[:<port>]/<file_pattern>[#transform=<trans_name>]'
           [, ...])
       | ('gpfdists://<filehost>[:<port>]/<file_pattern>[#transform=<trans_name>]'
           [, ...])
       | ('pxf://<path-to-data>?PROFILE=<profile_name>[&SERVER=<server_name>][&<custom-option>=<value>[...]]')
       | ('s3://<S3_endpoint>[:<port>]/<bucket_name>/[<S3_prefix>] [region=<S3-region>] [config=<config_file> | config_server=<url>]')
     [ON MASTER]
     FORMAT 'TEXT'
           [( [HEADER]
              [DELIMITER [AS] '<delimiter>' | 'OFF']
              [NULL [AS] '<null string>']
              [ESCAPE [AS] '<escape>' | 'OFF']
              [NEWLINE [ AS ] 'LF' | 'CR' | 'CRLF']
              [FILL MISSING FIELDS] )]
          | 'CSV'
           [( [HEADER]
              [QUOTE [AS] '<quote>']
              [DELIMITER [AS] '<delimiter>']
              [NULL [AS] '<null string>']
              [FORCE NOT NULL <column> [, ...]]
              [ESCAPE [AS] '<escape>']
              [NEWLINE [ AS ] 'LF' | 'CR' | 'CRLF']
              [FILL MISSING FIELDS] )]
          | 'CUSTOM' (Formatter=<formatter_specification>)
    [ ENCODING '<encoding>' ]
    [ [LOG ERRORS [PERSISTENTLY]] SEGMENT REJECT LIMIT <count>
      [ROWS | PERCENT] ]

CREATE [READABLE] EXTERNAL WEB [TEMPORARY | TEMP] TABLE <table_name>
   ( <column_name> <data_type> [, ...] | LIKE <other_table>)
      LOCATION ('http://<webhost>[:<port>]/<path>/<file>' [, ...])
    | EXECUTE '<command>' [ON ALL
                          | MASTER
                          | <number_of_segments>
                          | HOST ['<segment_hostname>']
                          | SEGMENT <segment_id> ]
      FORMAT 'TEXT'
            [( [HEADER]
               [DELIMITER [AS] '<delimiter>' | 'OFF']
               [NULL [AS] '<null string>']
               [ESCAPE [AS] '<escape>' | 'OFF']
               [NEWLINE [ AS ] 'LF' | 'CR' | 'CRLF']
               [FILL MISSING FIELDS] )]
           | 'CSV'
            [( [HEADER]
               [QUOTE [AS] '<quote>']
               [DELIMITER [AS] '<delimiter>']
               [NULL [AS] '<null string>']
               [FORCE NOT NULL <column> [, ...]]
               [ESCAPE [AS] '<escape>']
               [NEWLINE [ AS ] 'LF' | 'CR' | 'CRLF']
               [FILL MISSING FIELDS] )]
           | 'CUSTOM' (Formatter=<formatter_specification>)
     [ ENCODING '<encoding>' ]
    [ [LOG ERRORS [PERSISTENTLY]] SEGMENT REJECT LIMIT <count>
       [ROWS | PERCENT] ]

CREATE WRITABLE EXTERNAL [TEMPORARY | TEMP] TABLE <table_name>
    ( <column_name> <data_type> [, ...] | LIKE <other_table>)
     LOCATION('gpfdist://<outputhost>[:<port>]/<filename>[#transform=<trans_name>]'
          [, ...])
      | ('gpfdists://<outputhost>[:<port>]/<file_pattern>[#transform=<trans_name>]'
          [, ...])
      FORMAT 'TEXT'
               [( [DELIMITER [AS] '<delimiter>']
               [NULL [AS] '<null string>']
               [ESCAPE [AS] '<escape>' | 'OFF'] )]
          | 'CSV'
               [([QUOTE [AS] '<quote>']
               [DELIMITER [AS] '<delimiter>']
               [NULL [AS] '<null string>']
               [FORCE QUOTE <column> [, ...] | * ]
               [ESCAPE [AS] '<escape>'] )]

           | 'CUSTOM' (Formatter=<formatter_specification>)
    [ ENCODING '<write_encoding>' ]
    [ DISTRIBUTED BY ({<column> [<opclass>]}, [ ... ] ) | DISTRIBUTED RANDOMLY ]

CREATE WRITABLE EXTERNAL [TEMPORARY | TEMP] TABLE <table_name>
    ( <column_name> <data_type> [, ...] | LIKE <other_table>)
     LOCATION('s3://<S3_endpoint>[:<port>]/<bucket_name>/[<S3_prefix>] [region=<S3-region>] [config=<config_file> | config_server=<url>]')
      [ON MASTER]
      FORMAT 'TEXT'
               [( [DELIMITER [AS] '<delimiter>']
               [NULL [AS] '<null string>']
               [ESCAPE [AS] '<escape>' | 'OFF'] )]
          | 'CSV'
               [([QUOTE [AS] '<quote>']
               [DELIMITER [AS] '<delimiter>']
               [NULL [AS] '<null string>']
               [FORCE QUOTE <column> [, ...] | * ]
               [ESCAPE [AS] '<escape>'] )]

CREATE WRITABLE EXTERNAL WEB [TEMPORARY | TEMP] TABLE <table_name>
    ( <column_name> <data_type> [, ...] | LIKE <other_table> )
    EXECUTE '<command>'
    FORMAT 'TEXT'
               [( [DELIMITER [AS] '<delimiter>']
               [NULL [AS] '<null string>']
               [ESCAPE [AS] '<escape>' | 'OFF'] )]
          | 'CSV'
               [([QUOTE [AS] '<quote>']
               [DELIMITER [AS] '<delimiter>']
               [NULL [AS] '<null string>']
               [FORCE QUOTE <column> [, ...] | * ]
               [ESCAPE [AS] '<escape>'] )]
           | 'CUSTOM' (Formatter=<formatter_specification>)
    [ ENCODING '<write_encoding>' ]
    [ DISTRIBUTED BY ({<column> [<opclass>]}, [ ... ] ) | DISTRIBUTED RANDOMLY ]

Description

CREATE EXTERNAL TABLE or CREATE EXTERNAL WEB TABLE creates a new readable external table definition in Greengage DB. Readable external tables are typically used for fast, parallel data loading. Once an external table is defined, you can query its data directly (and in parallel) using SQL commands. For example, you can select, join, or sort external table data. You can also create views for external tables. DML operations (UPDATE, INSERT, DELETE, or TRUNCATE) are not allowed on readable external tables, and you cannot create indexes on readable external tables.

CREATE WRITABLE EXTERNAL TABLE or CREATE WRITABLE EXTERNAL WEB TABLE creates a new writable external table definition in Greengage DB. Writable external tables are typically used for unloading data from the database into a set of files or named pipes. Writable external web tables can also be used to output data to an executable program. Writable external tables can also be used as output targets for Greengage DB parallel MapReduce calculations. Once a writable external table is defined, data can be selected from database tables and inserted into the writable external table. Writable external tables only allow INSERT operations. SELECT, UPDATE, DELETE, or TRUNCATE are not allowed.

The main difference between regular external tables and external web tables is their data sources. Regular readable external tables access static flat files, whereas external web tables access dynamic data sources — either on a web server or by running OS commands or scripts.

See External tables overview for detailed information about working with external tables.

Parameters

Parameter Description

READABLE | WRITABLE

Specifies the type of external table, readable being the default. Readable external tables are used for loading data into Greengage DB. Writable external tables are used for unloading data

WEB

Creates a readable or writable external web table definition in Greengage DB. There are two forms of readable external web tables — those that access files via the HTTP protocol or those that access data by running OS commands. Writable external web tables output data to an executable program that can accept an input stream of data. External web tables are not rescannable during query execution.

The s3 protocol does not support external web tables. You can, however, create an external web table that runs a third-party tool to read data from or write data to S3 directly

TEMPORARY | TEMP

If specified, creates a temporary readable or writable external table definition in Greengage DB. Temporary external tables exist in a special schema; you cannot specify a schema name when you create the table. Temporary external tables are automatically dropped at the end of a session.

An existing permanent table with the same name is not visible to the current session while the temporary table exists, unless you reference the permanent table with its schema-qualified name

table_name

The name of the new external table

column_name

The name of a column to create in the external table definition. Unlike regular tables, external tables do not have column constraints or default values, so do not specify those

LIKE <other_table>

The LIKE clause specifies a table from which the new external table automatically copies all column names, data types, and distribution policy. If the original table specifies any column constraints or default column values, those will not be copied over to the new external table definition

data_type

The data type of the column

LOCATION ('<protocol>://[<host>[:<port>]]/<path>/<file>' [, …​])

If you use the pxf protocol to access an external data source, refer to the PXF documentation to learn more about creating external tables using the PXF protocol.

If you use the s3 protocol to read or write to S3, refer to the S3 section for additional information about the s3 protocol LOCATION clause syntax.

For readable external tables, specifies the URI of the external data source to be used to populate the external table or web table. Regular readable external tables allow the gpfdist or file protocols. External web tables allow the http protocol. If port is omitted, port 8080 is assumed for http and gpfdist protocols. If using the gpfdist protocol, the path is relative to the directory from which gpfdist is serving files (the directory specified when you started the gpfdist program). Also, gpfdist can use wildcards or other C-style pattern matching (for example, a whitespace character is [[:space:]]) to denote multiple files in a directory. For example:

'gpfdist://filehost:8081/*'
'gpfdist://masterhost/my_load_file'
'file://seghost1/dbfast1/external/myfile.txt'
'http://intranet.example.com/finance/expenses.csv'

For writable external tables, specifies the URI location of the gpfdist process or S3 storage that will collect data output from the Greengage DB segments and write it to one or more named files. For gpfdist, the path is relative to the directory from which gpfdist is serving files (the directory specified when you started the gpfdist program). If multiple gpfdist locations are listed, the segments sending data will be evenly divided across the available output locations. For example:

'gpfdist://outputhost:8081/data1.out',
'gpfdist://outputhost:8081/data2.out'

With two gpfdist locations listed as in the above example, half of the segments would send their output data to the data1.out file and the other half to the data2.out file.

With the #transform=<trans_name> option, you can specify a transformation to apply when loading or extracting data. <trans_name> is the name of the transform in the YAML configuration file you specify when running the gpfdist utility. For information about transformations, see Transform external data

ON MASTER

Restricts all table-related operations to the Greengage DB master segment. Permitted only on readable and writable external tables created with the s3 or custom protocols. The gpfdist, gpfdists, pxf, and file protocols do not support ON MASTER.

NOTE

Be aware of potential resource impacts when reading from or writing to external tables you create with the ON MASTER clause. You may encounter performance issues when you restrict table operations solely to the Greengage DB master segment.

EXECUTE '<command>' [ON …​]

Allowed for readable external web tables or writable external tables only. For readable external web tables, specifies the OS command to be run by the segment instances. The command can be a single OS command or a script. The ON clause is used to specify which segment instances will run the given command.

  • ON ALL is the default. The command will be run by every active (primary) segment instance on all segment hosts in the Greengage DB system. If the command runs a script, that script must reside in the same location on all of the segment hosts and be executable by the Greengage DB superuser (gpadmin).

  • ON MASTER runs the command on the master host only.

    NOTE

    Logging is not supported for external web tables when the ON MASTER clause is specified.

  • ON <number> means the command will be run by the specified number of segments. The particular segments are chosen randomly at runtime by the Greengage DB system. If the command runs a script, that script must reside in the same location on all of the segment hosts and be executable by the Greengage DB superuser (gpadmin).

  • HOST means the command will be run by one segment on each segment host (once per segment host), regardless of the number of active segment instances per host.

  • HOST <segment_hostname> means the command will be run by all active (primary) segment instances on the specified segment host.

  • SEGMENT <segment_id> means the command will be run only once by the specified segment. You can determine a segment instance’s ID by looking at the content number in the system catalog table gp_segment_configuration. The content ID of the Greengage DB master is always -1.

For writable external tables, the command specified in the EXECUTE clause must be prepared to have data piped into it. The command is executed on all segment instances that produce data. This behavior is equivalent to ON ALL for readable external web tables. However, the ON clause is not allowed for writable external tables

FORMAT 'TEXT | CSV' (<options>)

When the FORMAT clause identifies delimited text (TEXT) or comma-separated values (CSV) format, formatting options are similar to those available with the PostgreSQL COPY command. If the data in the file does not use the default column delimiter, escape character, null string and so on, you must specify the additional formatting options so that the data in the external file is read correctly by Greengage DB.

For more information on formatting external data, refer to Format external data.

If you use the pxf protocol to access an external data source, refer to the PXF documentation for information about using PXF

FORMAT 'CUSTOM' (formatter=<formatter_specification>)

Specifies a custom data format. The <formatter_specification> specifies the function to use to format the data, followed by comma-separated parameters to the formatter function. The length of the formatter specification, the string including Formatter=, can be up to approximately 50 kilobytes.

For more information on custom formats, refer to Use custom formats and protocols for external tables.

If you use the pxf protocol to access an external data source, refer to the PXF documentation for information about using PXF

DELIMITER

Specifies a single ASCII character that separates columns within each row (line) of data. The default is a tab character in TEXT mode, a comma in CSV mode. In TEXT mode for readable external tables, the delimiter can be set to OFF for special use cases in which unstructured data is loaded into a single-column table.

For the s3 protocol, the delimiter cannot be a newline character (\n) or a carriage return character (\r)

NULL

Specifies the string that represents a NULL value. The default is \N (backslash-N) in TEXT mode, and an empty value with no quotations in CSV mode. You might prefer an empty string even in TEXT mode for cases where you do not want to distinguish NULL values from empty strings. When using external and web tables, any data item that matches this string will be considered a NULL value.

As an example for the text format, this FORMAT clause can be used to specify that the string of two single quotes ('') is a NULL value:

FORMAT 'text' (delimiter ',' null '\'\'\'\'' )

ESCAPE

Specifies the single character that is used for C escape sequences (such as \n,\t,\100, and so on) and for escaping data characters that might otherwise be taken as row or column delimiters. Make sure to choose an escape character that is not used anywhere in your actual column data. The default escape character is a \ (backslash) for text-formatted files and a " (double quote) for CSV-formatted files, however it is possible to specify another character to represent an escape. It is also possible to deactivate escaping in text-formatted files by specifying the value 'OFF' as the escape value. This is very useful for data such as text-formatted web log data that has many embedded backslashes that are not intended to be escapes

NEWLINE

Specifies the newline used in your data files — LF (Line feed, 0x0A), CR (Carriage return, 0x0D), or CRLF (Carriage return plus line feed, 0x0D 0x0A). If not specified, a Greengage DB segment will detect the newline type by looking at the first row of data it receives and using the first newline type encountered

HEADER

For readable external tables, specifies that the first line in the data file is a header row (contains the names of the table columns) and should not be included as data for the table. If using multiple data source files, all files must have a header row.

For the s3 protocol, the column names in the header row cannot contain a newline character (\n) or a carriage return (\r).

The pxf protocol does not support the HEADER formatting option

QUOTE

Specifies the quotation character for CSV mode. The default is double-quote (")

FORCE NOT NULL

In CSV mode, processes each specified column as though it were quoted and hence not a NULL value. For the default null string in CSV mode (nothing between two delimiters), this causes missing values to be evaluated as zero-length strings

FORCE QUOTE

In CSV mode for writable external tables, forces quoting to be used for all non-NULL values in each specified column. If * is specified, then non-NULL values will be quoted in all columns. NULL output is never quoted

FILL MISSING FIELDS

In both TEXT and CSV mode for readable external tables, specifying FILL MISSING FIELDS will set missing trailing field values to NULL (instead of reporting an error) when a row of data has missing data fields at the end of a line or row. Blank rows, fields with a NOT NULL constraint, and trailing delimiters on a line will still report an error

ENCODING '<encoding>'

Character set encoding to use for the external table. Specify a string constant (such as 'SQL_ASCII'), an integer encoding number, or DEFAULT to use the default server encoding

LOG ERRORS [PERSISTENTLY]

This is an optional clause that can precede a SEGMENT REJECT LIMIT clause to log information about rows with formatting errors. The error log data is stored internally. If error log data exists for a specified external table, new data is appended to existing error log data. The error log data is not replicated to mirror segments.

The data is deleted when the external table is dropped unless you specify the keyword PERSISTENTLY. If the keyword is specified, the log data persists after the external table is dropped.

The error log data is accessed with the Greengage DB built-in SQL function gp_read_error_log(), or with the SQL function gp_read_persistent_error_log() if the PERSISTENTLY keyword is specified.

If you use the PERSISTENTLY keyword, you must install the functions that manage the persistent error log information.

See Notes for information about the error log information and built-in functions for viewing and managing error log information

SEGMENT REJECT LIMIT <count> [ROWS | PERCENT]

Runs a COPY FROM operation in single row error isolation mode. If the input rows have format errors, they will be discarded provided that the reject limit count is not reached on any Greengage DB segment instance during the load operation. The reject limit count can be specified as a number of rows (the default) or percentage of total rows (1-100). If PERCENT is used, each segment starts calculating the bad row percentage only after the number of rows specified by the parameter gp_reject_percent_threshold has been processed. The default for gp_reject_percent_threshold is 300 rows. Constraint errors such as violation of a NOT NULL, CHECK, or UNIQUE constraint will still be handled in "all-or-nothing" input mode. If the limit is not reached, all good rows will be loaded and any error rows discarded.

The limit for the number of initially rejected rows can be changed with the Greengage DB server configuration parameter gp_initial_bad_row_limit

DISTRIBUTED BY ({<column> [<opclass>]}, [ …​ ] )
DISTRIBUTED RANDOMLY

Used to declare the Greengage DB distribution policy for a writable external table. By default, writable external tables are distributed randomly. If the source table you are exporting data from has a hash distribution policy, defining the same distribution key column(s) and operator class(es), opclass, for the writable external table will improve unload performance by eliminating the need to move rows over the interconnect. When you issue an unload command such as INSERT INTO wex_table SELECT * FROM source_table, the rows that are unloaded can be sent directly from the segments to the output location if the two tables have the same hash distribution policy

Examples

Start the gpfdist file server program in the background on port 8081 serving files from directory /var/data/staging:

$ gpfdist -p 8081 -d /var/data/staging -l /home/<gpadmin>/log &

Create a readable external table named ext_customer using the gpfdist protocol and any text formatted files (*.txt) found in the gpfdist directory. The files are formatted with a pipe (|) as the column delimiter and an empty space as NULL. Also access the external table in single row error isolation mode:

CREATE EXTERNAL TABLE ext_customer
(
    id      int,
    name    text,
    sponsor text
)
    LOCATION ( 'gpfdist://filehost:8081/*.txt' )
    FORMAT 'TEXT' ( DELIMITER '|' NULL ' ')
    LOG ERRORS SEGMENT REJECT LIMIT 5;

Create the same readable external table definition as above, but with CSV formatted files:

CREATE EXTERNAL TABLE ext_customer
(
    id      int,
    name    text,
    sponsor text
)
    LOCATION ( 'gpfdist://filehost:8081/*.csv' )
    FORMAT 'CSV' ( DELIMITER ',' );

Create a readable external table named ext_expenses using the file protocol and several CSV formatted files that have a header row:

CREATE EXTERNAL TABLE ext_expenses
(
    name        text,
    date        date,
    amount      float4,
    category    text,
    description text
)
    LOCATION (
        'file://seghost1/dbfast/external/expenses1.csv',
        'file://seghost1/dbfast/external/expenses2.csv',
        'file://seghost2/dbfast/external/expenses3.csv',
        'file://seghost2/dbfast/external/expenses4.csv',
        'file://seghost3/dbfast/external/expenses5.csv',
        'file://seghost3/dbfast/external/expenses6.csv'
        )
    FORMAT 'CSV' ( HEADER );

Create a readable external web table that runs a script once per segment host:

CREATE EXTERNAL WEB TABLE log_output
(
    linenum int,
    message text
)
    EXECUTE '/var/load_scripts/get_log_data.sh' ON HOST
    FORMAT 'TEXT' (DELIMITER '|');

Create a writable external table named sales_out that uses gpfdist to write output data to a file named sales.out. The files are formatted with a pipe (|) as the column delimiter and an empty space as NULL:

CREATE WRITABLE EXTERNAL TABLE sales_out
(
    LIKE sales
)
    LOCATION ('gpfdist://etl1:8081/sales.out')
    FORMAT 'TEXT' ( DELIMITER '|' NULL ' ')
    DISTRIBUTED BY (txn_id);

Create a writable external web table that pipes output data received by the segments to an executable script named to_adreport_etl.sh:

CREATE WRITABLE EXTERNAL WEB TABLE campaign_out
(
    LIKE campaign
)
    EXECUTE '/var/unload_scripts/to_adreport_etl.sh'
    FORMAT 'TEXT' (DELIMITER '|');

Use the writable external table defined above to unload selected data:

INSERT INTO campaign_out
SELECT *
FROM campaign
WHERE customer_id = 123;

Notes

When you specify the LOG ERRORS clause, Greengage DB captures errors that occur while reading the external table data.

You can view and manage the captured error log data. The functions to manage log data depend on whether the data is persistent (the PERSISTENTLY keyword is used with the LOG ERRORS clause).

  • Functions that manage non-persistent error log data from external tables that were defined without the PERSISTENTLY keyword.

    • The built-in SQL function gp_read_error_log('table_name') displays error log information for an external table. This example displays the error log data from the external table ext_expenses:

      SELECT * from gp_read_error_log('ext_expenses');

      The function returns no data if you created the external table with the LOG ERRORS PERSISTENTLY clause, or if the external table does not exist.

    • The built-in SQL function gp_truncate_error_log('table_name') deletes the error log data for table_name. This example deletes the error log data captured from the external table ext_expenses:

      SELECT gp_truncate_error_log('ext_expenses');

      Dropping the table also deletes the table’s log data. The function does not truncate log data if the external table is defined with the LOG ERRORS PERSISTENTLY clause. The function returns FALSE if the table does not exist.

  • Functions that manage persistent error log data from external tables that were defined with the PERSISTENTLY keyword.

    NOTE

    The functions that manage persistent error log data from external tables are defined in the file $GPHOME/share/postgresql/contrib/gpexterrorhandle.sql. The functions must be installed in the databases that use persistent error log data from an external table. This psql command installs the functions into the database testdb.

    $ psql -d test -U gpadmin -f $GPHOME/share/postgresql/contrib/gpexterrorhandle.sql

    • The SQL function gp_read_persistent_error_log('table_name') displays persistent log data for an external table. The function returns no data if you created the external table without the PERSISTENTLY keyword. The function returns persistent log data for an external table even after the table has been dropped.

    • The SQL function gp_truncate_persistent_error_log('table_name') truncates persistent log data for a table. For persistent log data, you must manually delete the data. Dropping the external table does not delete persistent log data.

  • These items apply to both non-persistent and persistent error log data and the related functions:

    • The gp_read_* functions require the SELECT privilege on the table.

    • The gp_truncate_* functions require owner privilege on the table.

    • You can use the * wildcard character to delete error log information for existing tables in the current database. Specify the string *.* to delete all database error log information, including error log information that was not deleted due to previous database issues. If * is specified, database owner privilege is required. If *.* is specified, operating system superuser privilege is required. Non-persistent and persistent error log data must be deleted with their respective gp_truncate_* functions.

When multiple Greengage DB external tables are defined with the gpfdist, gpfdists, or file protocol and access the same named pipe on a Linux system, Greengage DB restricts access to the named pipe to a single reader. An error is returned if a second reader attempts to access the named pipe.

Compatibility

CREATE EXTERNAL TABLE is a Greengage DB extension. The SQL standard makes no provisions for external tables.

See also

Previous
CREATE EXTENSION
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CREATE FOREIGN DATA WRAPPER