Implement Merge Clause

MERGE either matches existing nodes and binds them, or
it creates new data and binds that. It’s like a
combination  of  MATCH  and  CREATE  that  additionally
allows  you  to  specify  what  happens  if  the  data
was matched or created.

For example, you can specify that the graph must contain
a node for a user with a certain name. If there isn’t a
node with the correct name, a new node will be created
and its name property set.

When  using  MERGE  on  full patterns, the behavior is
that either the whole pattern matches, or the whole
pattern is created. MERGE will not partially use existing
patterns—it’s all or nothing. If partial matches are
needed, this can be accomplished by splitting a pattern up
into multiple MERGE clauses.

As with MATCH, MERGE can match multiple occurrences of a
pattern. If there are multiple matches, they will all be
passed on to later stages of the query.

Makefile

@@ -27,6 +27,7 @@ OBJS = src/backend/age.o \
        src/backend/commands/graph_commands.o \
        src/backend/commands/label_commands.o \
        src/backend/executor/cypher_create.o \
+       src/backend/executor/cypher_merge.o \
        src/backend/executor/cypher_set.o \
        src/backend/executor/cypher_utils.o \
        src/backend/nodes/ag_nodes.o \
@@ -79,6 +80,7 @@ REGRESS = scan \
           cypher_with \
           cypher_vle \
           cypher_union \
+          cypher_merge \
           drop
 
 srcdir=`pwd`

age--0.7.0.sql

@@ -3130,6 +3130,11 @@ RETURNS void
 LANGUAGE c
 AS 'MODULE_PATHNAME';
 
+CREATE FUNCTION ag_catalog._cypher_merge_clause(internal)
+RETURNS void
+LANGUAGE c
+AS 'MODULE_PATHNAME';
+
 --
 -- query functions
 --