A foreign data wrapper is a specification to access remote data, also called SQL management of External data (SQL/MED) . It was added to SQL standard in 2003. This feature has been developing by PostgreSQL.
So, in SQL/MED, there is a table on a remote server called foreign table. Now PostgreSQL foreign Data wrappers (FDW) are that use SQL/MED to manage foreign tables.
Accessing these foreign tables on remote server:
We can access these foreign tables on the remote server after installing some specific software. Lets say we have two different remote servers, namely, postgresql and mysql. Which have foreign_pg_tbl ad foreign_my_tbl, respectively. We can access these foreign table from the remote server to our local server by using and issuing the SELECT queries as follows.
To access table from remote postgresql Server.
Localdb=# -- foreign_pg_tbl is on the remote postgresql server.
localdb-# SELECT count(*) FROM foreign_pg_tbl;
count
output:
2000
To access table from remote mysql Server.
Localdb=# -- foreign_my_tbl is on the remote mysql server.
localdb-# SELECT count(*) FROM foreign_my_tbl;
count
10,000
we can also use the join operations on the foreign tables just like we use it on the local tables stored.
For example
Localdb=# SELECT count(*) FROM foreign_pg_tbl AS p, foreign_my_tbl AS m WHERE p.id = m.id;
Count
10,000
That's just a basic overview about FDWs. In the coming sections we will explore more about it๐๐ป. If you are interested and want to read more about this you can check references.
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