Creating a GDS Foreign Table

The source data information and GDS access information are configured in a foreign table. Then, GaussDB(DWS) can import data from a data server to a database table based on the configuration in the foreign table.

Procedure

  1. Collect source data information and GDS access information.

    You need to collect the following source data information:

    • format: format of the data to be imported. Only data in CSV, TEXT, or FIXED format can be imported using GDS in parallel.
    • header: whether a source data file has a header. This parameter is set only for files in CSV or FIXED format.
    • delimiter: delimiter in the source data file. For example, it can be a comma (,).
    • encoding: encoding format of the data source file. Assume that the encoding format is UTF-8.
    • eol: line break character in the data file. It can be a default character, such as 0x0D0A or 0X0A, or a customized line break character, such as a string: !@#. This parameter can be set only for TEXT import.
    • For details about more source data information configured in a foreign table, see data format parameters.

    You need to collect the following GDS access information:

    location: GDS URL. GDS information in Installing, Configuring, and Starting GDS is used as an example. In non-SSL mode, location is set to gsfs://192.168.0.90:5000//input_data/. In SSL mode, location is set to gsfss://192.168.0.90:5000//input_data/. 192.168.0.90:5000 indicates the IP address and port number of GDS. input_data indicates the path of data source files managed by GDS. Replace the values as required.

  2. Design an error tolerance mechanism for data import.

    GaussDB(DWS) supports the following error tolerance in data import:
    • fill_missing_fields: When the last column in a row of the source data file is empty, this parameter specifies whether to report an error or set this field in the row to NULL.
      Valid value: true, on, false, and off.
      • If this parameter is set to true or on and the last column of a data row in a source data file is lost, the column will be replaced with null and no error message will be generated.
      • If this parameter is set to false or off and the last column of a data row in a source data file is lost, the following error information will be displayed:
        missing data for column "tt"

      Default value: false or off

    • ignore_extra_data: When the number of columns in the source data file is greater than that specified in the foreign table, this parameter specifies whether to report an error or ignore the extra columns.

      Value range: true/on, false/off.

      • When this parameter is true or on and the number of data source files exceeds the number of foreign table columns, excessive columns will be ignored.
      • If the parameter is set to false or off, and the number of data source files exceeds the number of foreign table columns, the following error information will be displayed:
        1
        extra data after last expected column

      Default value: false or off

    • per node reject_limit: This parameter specifies the number of data format errors allowed on each DN. If the number of errors recorded in the error table on a DN exceeds the specified value, the import will fail and an error message will be reported. You can also set it to unlimited.
    • compatible_illegal_chars

      Enables or disables fault tolerance on invalid characters during their import and export. This syntax takes effect for read-only and write-only foreign tables.

      Only clusters of version 8.1.3.331 or later support export fault tolerance.

      Value range: true, on, false, and off. The default value is false or off.

      • If the parameter is true or on, invalid characters are tolerated and imported to or exported from the database after being converted.
      • If the parameter is false or off, and an error occurs when there are invalid characters, the import or export will be interrupted.

      The rule of error tolerance when you import or export invalid characters is as follows:

      • \0 is converted to a space.
      • Other invalid characters are converted to question marks.
      • If compatible_illegal_chars is set to true or on, the database will convert and accept the invalid characters. If NULL, DELIMITER, QUOTE, and ESCAPE are set to a spaces or question marks. Errors like "illegal chars conversion may confuse COPY escape 0x20" will be displayed to prompt user to modify parameter values that cause confusion, preventing import and export errors.
      • Enabling error tolerance for foreign table export will result in invalid characters being exported as question marks (?), which can lead to inconsistencies between the exported and original data when imported back into the GaussDB(DWS) database.
    • error_table_name: This parameter specifies the name of the table that records data format errors. After the parallel import, you can query this table for error details.
    • remote log 'name': This parameter specifies whether to store data format errors in files on the GDS server. name is the prefix of the error data file.
    • For details about more error tolerance parameters, see error tolerance parameters.

  3. After connecting to the database using gsql or Data Studio, create a GDS foreign table based on the collected and design information.

    For example:

     1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
    CREATE FOREIGN TABLE foreign_tpcds_reasons
(
  r_reason_sk integer not null,
  r_reason_id char(16) not null,
  r_reason_desc char(100)
)
 SERVER gsmpp_server
 OPTIONS 
(
LOCATION 'gsfs://192.168.0.90:5000/* | gsfs://192.168.0.91:5000/*', 
FORMAT 'CSV' ,
DELIMITER ',',
ENCODING 'utf8',
HEADER 'false',
FILL_MISSING_FIELDS 'true',
IGNORE_EXTRA_DATA 'true'
)
LOG INTO product_info_err 
PER NODE REJECT LIMIT 'unlimited';

    The following describes information in the preceding command:

    • The columns specified in the foreign table must be the same as those in the target table.
    • Retain the value gsmpp_server for SERVER.
    • Set location based on the GDS access information collected in 1. If SSL is used, replace gsfs with gsfss.
    • Set FORMAT, DELIMITER, ENCODING, and HEADER based on the source data information collected in 1.
    • Set FILL_MISSING_FIELDS, IGNORE_EXTRA_DATA, LOG INTO, and PER NODE REJECT LIMIT based on the error tolerance mechanism designed in 2. LOG INTO specifies the name of the error table.

    For details about the CREATE FOREIGN TABLE syntax, see CREATE FOREIGN TABLE (for GDS Import and Export).

Example

For more examples, see Example of Importing Data Using GDS.

Parent topic: Using GDS to Import Data from a Remote Server