Add support for MERGE SQL command
 
MERGE performs actions that modify rows in the target table using a
source table or query. MERGE provides a single SQL statement that can
conditionally INSERT/UPDATE/DELETE rows -- a task that would otherwise
require multiple PL statements.  For example,
 
MERGE INTO target AS t
USING source AS s
ON t.tid = s.sid
WHEN MATCHED AND t.balance > s.delta THEN
  UPDATE SET balance = t.balance - s.delta
WHEN MATCHED THEN
  DELETE
WHEN NOT MATCHED AND s.delta > 0 THEN
  INSERT VALUES (s.sid, s.delta)
WHEN NOT MATCHED THEN
  DO NOTHING;
 
MERGE works with regular tables, partitioned tables and inheritance
hierarchies, including column and row security enforcement, as well as
support for row and statement triggers and transition tables therein.
 
MERGE is optimized for OLTP and is parameterizable, though also useful
for large scale ETL/ELT. MERGE is not intended to be used in preference
to existing single SQL commands for INSERT, UPDATE or DELETE since there
is some overhead.  MERGE can be used from PL/pgSQL.
 
MERGE does not support targetting updatable views or foreign tables, and
RETURNING clauses are not allowed either.  These limitations are likely
fixable with sufficient effort.  Rewrite rules are also not supported,
but it's not clear that we'd want to support them.
 
Author: Pavan Deolasee <[email protected]>
Author: Álvaro Herrera <[email protected]>
Author: Amit Langote <[email protected]>
Author: Simon Riggs <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Reviewed-by: Andres Freund <[email protected]> (earlier versions)
Reviewed-by: Peter Geoghegan <[email protected]> (earlier versions)
Reviewed-by: Robert Haas <[email protected]> (earlier versions)
Reviewed-by: Japin Li <[email protected]>
Reviewed-by: Justin Pryzby <[email protected]>
Reviewed-by: Tomas Vondra <[email protected]>
Reviewed-by: Zhihong Yu <[email protected]>
Discussion: https://postgr.es/m/CANP8[email protected]
Discussion: https://postgr.es/m/CAH2-WzkJdBuxj9PO[email protected]
Discussion: https://postgr.es/m/[email protected]