CREATE EXTERNAL TABLE

Description

or CREATE EXTERNAL WEB TABLE creates a new readable external table definition in Greenplum Database. 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 Greenplum Database. 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 Greenplum 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 executing OS commands or scripts.

See Working with External Data for detailed information about working with external tables.

READABLE | WRITABLE

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

WEB

Creates a readable or writable external web table definition in Greenplum Database. There are two forms of readable external web tables – those that access files via the http:// protocol or those that access data by executing 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 executes 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 Greenplum Database. 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 Greenplum 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 gphdfs protocol (deprecated) to read or write a file to a Hadoop file system (HDFS), refer to Specify gphdfs Protocol in an External Table Definition (Deprecated) for additional information about the gphdfs protocol LOCATION clause syntax.

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

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

For readable external tables, specifies the URI of the external data source(s) 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, and port 9000 for the gphdfs protocol (deprecated). 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 protocol that will collect data output from the Greenplum 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 option #transform=trans\_name, you can specify a transform to apply when loading or extracting data. The trans_name is the name of the transform in the YAML configuration file you specify with the you run the gpfdist utility. For information about specifying a transform, see in the Greenplum Utility Guide.

ON MASTER

Restricts all table-related operations to the Greenplum master segment. Permitted only on readable and writable external tables created with the s3 or custom protocols. The gpfdist, gpfdists, gphdfs (deprecated), 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 Greenplum 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 executed 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 execute the given command.

• ON ALL is the default. The command will be executed by every active (primary) segment instance on all segment hosts in the Greenplum Database system. If the command executes a script, that script must reside in the same location on all of the segment hosts and be executable by the Greenplum 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 executed by the specified number of segments. The particular segments are chosen randomly at runtime by the Greenplum Database system. If the command executes a script, that script must reside in the same location on all of the segment hosts and be executable by the Greenplum superuser (gpadmin).
• HOST means the command will be executed 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 executed by all active (primary) segment instances on the specified segment host.
• SEGMENT segment_id means the command will be executed 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 Greenplum Database master is always -1.

For writable external tables, the command specified in the EXECUTE clause must be prepared to have data piped into it. Since all segments that have data to send will write their output to the specified command or program, the only available option for the ON clause is ON ALL.

When the FORMAT clause identfies delimited text (TEXT) or comma separated values (CSV) format, formatting options are similar to those available with the PostgreSQL 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 Greenplum Database. For information about using a custom format, see “Loading and Unloading Data” in the Greenplum Database Administrator Guide.

The AVRO and formats are supported only when you specify the gphdfs protocol (deprecated). Refer to Accessing HDFS Data with gphdfs (Deprecated) for detailed information about the gphdfs protocol FORMAT clause syntax.

If you use the pxf protocol to access an external data source, refer to 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 50K bytes.

If you use the pxf protocol to access an external data source, refer to Accessing External Data with PXF for information about using PXF.

For general information about using a custom format, see “Loading and Unloading Data” in the Greenplum Database Administrator Guide.

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.

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 Greenplum Database 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(s) 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 client encoding. See .

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.

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 Greenplum segment instance during the load operation. The reject limit count can be specified as 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.

Note: When reading an external table, Greenplum Database limits the initial number of rows that can contain formatting errors if the SEGMENT REJECT LIMIT is not triggered first or is not specified. If the first 1000 rows are rejected, the COPY operation is stopped and rolled back.

The limit for the number of initial rejected rows can be changed with the Greenplum Database server configuration parameter gp_initial_bad_row_limit. See for information about the parameter.

DISTRIBUTED BY (column, [ … ] )

DISTRIBUTED RANDOMLY

Used to declare the Greenplum Database 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) 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 :

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 executes a script once per segment host:

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 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;

When you specify the LOG ERRORS clause, Greenplum Database captures errors that occur while reading the external table data. For information about the error log format, see Viewing Bad Rows in the Error Log.

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.

  • 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 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 super-user privilege is required. Non-persistent and persistent error log data must be deleted with their respective gp_truncate_* functions.

When multiple Greenplum Database external tables are defined with the gpfdist, gpfdists, or file protocol and access the same named pipe a Linux system, Greenplum Database 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

is a Greenplum Database extension. The SQL standard makes no provisions for external tables.

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