Implement PGXC with FDW

We can implement postgres-XC feature using FDW framework.

Outline:

• Create class catalog for fdw (namely, pgxc_fdw).

• Implement new fdw driver called pgxc_fdw.

• pgxc_fdw takes care of both sharded tables and replicated tables.

• Use FDW "server" to define the group of database servers.

• Each shard is assigned to each database in one of a database in "server".

• Replicated table will be defined in all the databases in "server".

• • Should define "primary databsae" somewhere in this catalog (table-specific or server-specific).

• "Server" catalog contains server host, port and connection striong.

• Implement full set of fdw functions so that planner call it for pushdown.

• Actual connection and data fetch can be done just inside fdw drivers.

• In similar wary, we can mplement aggregate pushdown.

• With parallel worker mechanism, we can implement MPP with no sweat.

• Consider semi-join pushdown added in PG17: https://qiita.com/t-hiroshige/items/5e02e4f1297957a6809d*

• In the case of DML against replicated tables, issue DML to the primary node, and then issue to remiaining nodes using parallel worker mechanism.

• Implement connection pooler for performance. Pooler belongs to "server", sharing credentials and session parameters.

• Implement consistent write using 2PC.

• Distributed deadlock detection should be implemented as separate feature (need PG kernel modification).

• Distributed TX management should be implemented as a separete feature, which also needs PG kernel modification.