Create  readable/writable external/ external web table command in Greenplum

• 1. CREATE EXTERNAL TABLE 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. 

  2. DML operations (UPDATE, INSERT, DELETE, or TRUNCATE) are not allowed on readable external tables, and you cannot create indexes on readable external tables.

  3. 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. 

  4. Writable external tables only allow INSERT operations – SELECT, UPDATE, DELETE or TRUNCATE are not allowed.

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

  6. The FORMAT clause is used to describe how the external table files are formatted. Valid file formats are delimited text (TEXT) for all protocols and comma separated values (CSV) format for gpfdist and file protocols, similar to the formatting options 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 Greenplum Database.

CREATE [READABLE] EXTERNAL TABLE table_name

( column_name data_type [, ...] | LIKE other_table )

LOCATION ('file://seghost[:port]/path/file' [, ...])

| ('gpfdist://filehost[:port]/file_pattern[#transform]'

| ('gpfdists://filehost[:port]/file_pattern[#transform]'

[, ...])

| ('gphdfs://hdfs_host[:port]/path/file')

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 specifications>)

[ ENCODING 'encoding' ]

[ [LOG ERRORS INTO error_table] SEGMENT REJECT LIMIT count

[ROWS | PERCENT] ]

CREATE [READABLE] EXTERNAL WEB 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 specifications>)

[ ENCODING 'encoding' ]

[ [LOG ERRORS INTO error_table] SEGMENT REJECT LIMIT count

[ROWS | PERCENT] ]

CREATE WRITABLE EXTERNAL TABLE table_name

( column_name data_type [, ...] | LIKE other_table )

LOCATION('gpfdist://outputhost[:port]/filename[#transform]'

| ('gpfdists://outputhost[:port]/file_pattern[#transform]'

[, ...])

| ('gphdfs://hdfs_host[:port]/path')

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 specifications>)

[ ENCODING 'write_encoding' ]

[ DISTRIBUTED BY (column, [ ... ] ) | DISTRIBUTED RANDOMLY ]

CREATE WRITABLE EXTERNAL WEB TABLE table_name

( column_name data_type [, ...] | LIKE other_table )

EXECUTE 'command' [ON ALL]

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 specifications>)

[ ENCODING 'write_encoding' ]

[ DISTRIBUTED BY (column, [ ... ] ) | DISTRIBUTED RANDOMLY ]