Foreign Scan

Description

In GaussDB, foreign scan is a scanner used to access foreign data sources. It can process data in foreign data sources as tables in relational databases to implement query and operations on foreign data sources. In GaussDB, foreign scan can be implemented by extending APIs. You can compile your own extension programs to access and operate foreign data sources. In addition, GaussDB provides some common extension programs, such as foreign data wrapper (FDW), which can be used to access data in other relational databases.

Typical Scenarios

• Foreign data source accessing: When you need to access foreign data sources, you can use foreign scan to read data from the foreign data source.
• Data integration: When data from multiple data sources needs to be integrated, you can use a foreign scan to process the data as a table in a relational database.
• Data migration: When data needs to be migrated from one data source to another, you can use a foreign scan to read data from the source data source and insert the data into the target data source.

Examples

Example: Access a foreign data source.

-- Prepare data.
gaussdb=# SET enable_extension = true;
gaussdb=# CREATE EXTENSION file_fdw; 
gaussdb=# CREATE SERVER foo FOREIGN DATA WRAPPER file_fdw; 
gaussdb=# CREATE FOREIGN TABLE ft (a int, b text) SERVER foo OPTIONS ("format" 'csv', "filename" '/gaussdb/username/data.csv', "null" 'NULL');

-- Execution result.
gaussdb=# EXPLAIN SELECT * FROM ft; 
                         QUERY PLAN                         
------------------------------------------------------------
 Foreign Scan on ft  (cost=0.00..138.00 rows=1280 width=36)
   Foreign File: /gaussdb/username/data.csv
(2 rows)

-- Drop.
gaussdb=# DROP FOREIGN TABLE ft;
gaussdb=# DROP SERVER foo;
gaussdb=# DROP EXTENSION file_fdw;

In the preceding example, the output of the foreign scan operator is as follows.