Add documentation for MergeUniqueNode (#1428)

Co-authored-by: Stefano Ottolenghi <[email protected]>

Author: pontusmelke

modules/ROOT/pages/planning-and-tuning/operators/operators-detail.adoc

@@ -6056,6 +6056,54 @@ Total database accesses: 15, total allocated memory: 2232
 
 ======
 
+[[query-plan-merge-unique-node]]
+=== Merge Unique Node
+
+////
+[source, cypher, role=test-setup]
+----
+CREATE CONSTRAINT team_name IF NOT EXISTS FOR (t:Team) REQUIRE (t.name) IS UNIQUE
+----
+////
+
+The `MergeUniqueNode` operator is similar to the xref:planning-and-tuning/operators/operators-detail.adoc#query-plan-merge[`Merge`] operator, but is used when there is a property uniqueness constraint on the property used in the `MERGE` statement.
+
+.MergeUniqueNode
+======
+
+.Query
+[source, cypher]
+----
+PROFILE
+MERGE (t:Team {name: 'Engineering'})
+----
+
+.Query Plan
+[role="queryplan", subs="attributes+"]
+----
+Planner COST
+
+Runtime PIPELINED
+
+Runtime version {neo4j-version}
+
+Batch size 128
+
++------------------+----+---------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+--------------+
+| Operator         | Id | Details                                           | Estimated Rows | Rows | DB Hits | Memory (Bytes) | Page Cache Hits/Misses | Time (ms) | Pipeline            | Indexes Used |
++------------------+----+---------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+--------------+
+| +ProduceResults  |  0 |                                                   |              1 |    0 |       0 |              0 |                        |           |                     |              |
+| |                +----+---------------------------------------------------+----------------+------+---------+----------------+                        |           |                     +--------------+
+| +EmptyResult     |  1 |                                                   |              1 |    0 |       0 |                |                        |           |                     |              |
+| |                +----+---------------------------------------------------+----------------+------+---------+----------------+                        |           |                     +--------------+
+| +MergeUniqueNode |  2 | UNIQUE t:Team(name) WHERE name = $autostring_0,   |              1 |    1 |       2 |            248 |                    1/0 |     2.922 | Fused in Pipeline 0 | team_name: 1 |
++------------------+----+---------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+--------------+
+
+Total database accesses: 2, total allocated memory: 312
+----
+
+======
+
 
 [[query-plan-locking-merge]]
 === Locking Merge