Document lower bound restriction for shortestPath() and allShortestPa…

…th() functions (#1146)

Document that the shortestPath() and allShortestPath() functions can
only have a lower bound of 0 or 1 for its variable length pattern.

This is not allowed:
` MATCH p=shortestPath((a:A)-[:R*2..]->(b:B)) RETURN 1`

A lower bound can be specified using a filter, but might lead to
executing a possibly very slow fallback plan.
` MATCH p=shortestPath((a:A)-[:R*]->(b:B)) WHERE length(p)>2 RETURN 1`

The possibly very slow fallback plan is this part:
```
| | +Top                   |  3 | anon_1 ASC LIMIT 1                                    |              1 | In Pipeline 6       |
| | |                      +----+-------------------------------------------------------+----------------+---------------------+
| | +Projection            |  4 | length(p) AS anon_1                                   |              1 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +Filter                |  5 | length(p) > $autoint_0                                |              1 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +Projection            |  6 | (a)-[anon_0*]->(b) AS p                               |              4 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +VarLengthExpand(Into) |  7 | (a)-[anon_0:R*]->(b)                                  |              4 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +Argument              |  8 | a, b                                                  |            100 | Fused in Pipeline 5 |
| |                        +----+-------------------------------------------------------+----------------+---------------------+

```

Within this query plan:
```
+--------------------------+----+-------------------------------------------------------+----------------+---------------------+
| Operator                 | Id | Details                                               | Estimated Rows | Pipeline            |
+--------------------------+----+-------------------------------------------------------+----------------+---------------------+
| +ProduceResults          |  0 | `1`                                                   |             30 |                     |
| |                        +----+-------------------------------------------------------+----------------+                     |
| +Projection              |  1 | $autoint_1 AS `1`                                     |             30 |                     |
| |                        +----+-------------------------------------------------------+----------------+                     |
| +AntiConditionalApply    |  2 |                                                       |             30 | Fused in Pipeline 7 |
| |\                       +----+-------------------------------------------------------+----------------+---------------------+
| | +Top                   |  3 | anon_1 ASC LIMIT 1                                    |              1 | In Pipeline 6       |
| | |                      +----+-------------------------------------------------------+----------------+---------------------+
| | +Projection            |  4 | length(p) AS anon_1                                   |              1 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +Filter                |  5 | length(p) > $autoint_0                                |              1 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +Projection            |  6 | (a)-[anon_0*]->(b) AS p                               |              4 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +VarLengthExpand(Into) |  7 | (a)-[anon_0:R*]->(b)                                  |              4 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +Argument              |  8 | a, b                                                  |            100 | Fused in Pipeline 5 |
| |                        +----+-------------------------------------------------------+----------------+---------------------+
| +Apply                   |  9 |                                                       |            100 |                     |
| |\                       +----+-------------------------------------------------------+----------------+---------------------+
| | +Optional              | 10 | a, b                                                  |            100 | In Pipeline 4       |
| | |                      +----+-------------------------------------------------------+----------------+---------------------+
| | +ShortestPath          | 11 | p = (a)-[anon_0:R*]->(b) WHERE length(p) > $autoint_0 |             30 |                     |
| | |                      +----+-------------------------------------------------------+----------------+                     |
| | +Argument              | 12 | a, b                                                  |            100 | Fused in Pipeline 3 |
| |                        +----+-------------------------------------------------------+----------------+---------------------+
| +CartesianProduct        | 13 |                                                       |            100 | In Pipeline 2       |
| |\                       +----+-------------------------------------------------------+----------------+---------------------+
| | +NodeByLabelScan       | 14 | b:B                                                   |             10 | In Pipeline 1       |
| |                        +----+-------------------------------------------------------+----------------+---------------------+
| +NodeByLabelScan         | 15 | a:A                                                   |             10 | In Pipeline 0       |
+--------------------------+----+-------------------------------------------------------+----------------+---------------------+
```

Better is to use the keyword-based SHORTEST.
`MATCH p = SHORTEST 1 (a:A)-[:R]->{2,}(b:B) RETURN 1`

---------

Co-authored-by: Jens Pryce-Åklundh <[email protected]>

modules/ROOT/pages/patterns/reference.adoc

@@ -1372,6 +1372,7 @@ They are similar to `SHORTEST 1` and `ALL SHORTEST`, but with several difference
 * The path pattern is passed as an argument to the functions.
 * The path pattern is limited to a single relationship pattern.
 * To return results where the first and last node in the path are the same requires a change to the configuration setting link:{neo4j-docs-base-uri}/operations-manual/{page-version}/configuration/configuration-settings/#config_dbms.cypher.forbid_shortestpath_common_nodes[`dbms.cypher.forbid_shortestpath_common_nodes`].
+* The minimum path length, also called the lower bound of the variable length relationship pattern, should be 0 or 1.
 
 Both functions will continue to be available, but they are not xref:appendix/gql-conformance/index.adoc[GQL conformant].
 
@@ -1414,7 +1415,7 @@ shortestPath((:A)-->+(:B))
 [[shortest-functions-rules-path-pattern-length]]
 ==== Path pattern length
 
-There must be exactly one relationship pattern in the path pattern.
+There must be exactly one relationship pattern in the path pattern, and the lower bound should be 0 or 1.
 
 .Allowed
 [source]
@@ -1427,6 +1428,8 @@ shortestPath((a)-[:R*1..5]-(b))
 ----
 shortestPath((a)-[:R*1..5]-(b)-->(:X))
 
+shortestPath((a)-[:R*2..5]-(b))
+
 shortestPath((:A))
 
 allShortestPaths((a:A)-[:S*]->(:B), (a)-[:R*1..3]->(:C))