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 <pavan.deolasee@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Author: Amit Langote <amitlangote09@gmail.com>
Author: Simon Riggs <simon.riggs@enterprisedb.com>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: Andres Freund <andres@anarazel.de> (earlier versions)
Reviewed-by: Peter Geoghegan <pg@bowt.ie> (earlier versions)
Reviewed-by: Robert Haas <robertmhaas@gmail.com> (earlier versions)
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Discussion: https://postgr.es/m/CANP8+jKitBSrB7oTgT9CY2i1ObfOt36z0XMraQc+Xrz8QB0nXA@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-WzkJdBuxj9PO=2QaO9-3h3xGbQPZ34kJH=HukRekwM-GZg@mail.gmail.com
Discussion: https://postgr.es/m/20201231134736.GA25392@alvherre.pgsql

1 files changed, 91 insertions, 0 deletions

diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c
index 74823e8437a..0bd99acf836 100644
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -133,6 +133,86 @@ static Node *find_jointree_node_for_rel(Node *jtnode, int relid);
 
 
 /*
+ * transform_MERGE_to_join
+ *		Replace a MERGE's jointree to also include the target relation.
+ */
+void
+transform_MERGE_to_join(Query *parse)
+{
+	RangeTblEntry *joinrte;
+	JoinExpr   *joinexpr;
+	JoinType	jointype;
+	int			joinrti;
+	List	   *vars;
+
+	if (parse->commandType != CMD_MERGE)
+		return;
+
+	/* XXX probably bogus */
+	vars = NIL;
+
+	/*
+	 * When any WHEN NOT MATCHED THEN INSERT clauses exist, we need to use an
+	 * outer join so that we process all unmatched tuples from the source
+	 * relation.  If none exist, we can use an inner join.
+	 */
+	if (parse->mergeUseOuterJoin)
+		jointype = JOIN_RIGHT;
+	else
+		jointype = JOIN_INNER;
+
+	/* Manufacture a join RTE to use. */
+	joinrte = makeNode(RangeTblEntry);
+	joinrte->rtekind = RTE_JOIN;
+	joinrte->jointype = jointype;
+	joinrte->joinmergedcols = 0;
+	joinrte->joinaliasvars = vars;
+	joinrte->joinleftcols = NIL;	/* MERGE does not allow JOIN USING */
+	joinrte->joinrightcols = NIL;	/* ditto */
+	joinrte->join_using_alias = NULL;
+
+	joinrte->alias = NULL;
+	joinrte->eref = makeAlias("*MERGE*", NIL);
+	joinrte->lateral = false;
+	joinrte->inh = false;
+	joinrte->inFromCl = true;
+	joinrte->requiredPerms = 0;
+	joinrte->checkAsUser = InvalidOid;
+	joinrte->selectedCols = NULL;
+	joinrte->insertedCols = NULL;
+	joinrte->updatedCols = NULL;
+	joinrte->extraUpdatedCols = NULL;
+	joinrte->securityQuals = NIL;
+
+	/*
+	 * Add completed RTE to pstate's range table list, so that we know its
+	 * index.
+	 */
+	parse->rtable = lappend(parse->rtable, joinrte);
+	joinrti = list_length(parse->rtable);
+
+	/*
+	 * Create a JOIN between the target and the source relation.
+	 */
+	joinexpr = makeNode(JoinExpr);
+	joinexpr->jointype = jointype;
+	joinexpr->isNatural = false;
+	joinexpr->larg = (Node *) makeNode(RangeTblRef);
+	((RangeTblRef *) joinexpr->larg)->rtindex = parse->resultRelation;
+	joinexpr->rarg = linitial(parse->jointree->fromlist);	/* original join */
+	joinexpr->usingClause = NIL;
+	joinexpr->join_using_alias = NULL;
+	/* The quals are removed from the jointree and into this specific join */
+	joinexpr->quals = parse->jointree->quals;
+	joinexpr->alias = NULL;
+	joinexpr->rtindex = joinrti;
+
+	/* Make the new join be the sole entry in the query's jointree */
+	parse->jointree->fromlist = list_make1(joinexpr);
+	parse->jointree->quals = NULL;
+}
+
+/*
  * replace_empty_jointree
  *		If the Query's jointree is empty, replace it with a dummy RTE_RESULT
  *		relation.
@@ -2058,6 +2138,17 @@ perform_pullup_replace_vars(PlannerInfo *root,
 		 * can't contain any references to a subquery.
 		 */
 	}
+	if (parse->mergeActionList)
+	{
+		foreach(lc, parse->mergeActionList)
+		{
+			MergeAction *action = lfirst(lc);
+
+			action->qual = pullup_replace_vars(action->qual, rvcontext);
+			action->targetList = (List *)
+				pullup_replace_vars((Node *) action->targetList, rvcontext);
+		}
+	}
 	replace_vars_in_jointree((Node *) parse->jointree, rvcontext,
 							 lowest_nulling_outer_join);
 	Assert(parse->setOperations == NULL);