Before a Cypher query is executed, it is converted into an internal form suitable for execution, known as a plan. A plan is a tree-like data structure describing a pipeline of operations which will be performed on the database in order to yield the results for a given query. Every node within a plan is known as a logical operator and describes a particular operation.
Because a plan represents a pipeline, the logical operators are iteratively executed as data passes from one logical operator to the other. Every logical operator pulls data from the logical operator(s) preceding it, processes it and passes it onto the logical operator next in the pipeline for further processing.
EXPLAIN operator, it is possible for the user to inspect the
produced plan and gain insight into the execution of a query. Currently, the
various logical operators aren't fully documented as their behavior is subject
to change. However, the behavior of most of them can be deduced from their name.
In the future, additional information might be added to the output of the
As an example, let's inspect the plan produced for a simple query:
The output of the
EXPLAIN query is a representation of the produced plan. Every
logical operator within the plan starts with an asterisk character (
*) and is
followed by its name (and sometimes additional information). The execution of
the query proceeds iteratively (generating one entry of the result set at a
time), with data flowing from the bottom-most logical operator(s) (the start of
the pipeline) to the top-most logical operator(s) (the end of the pipeline).
In the example above, the resulting plan is a pipeline of 3 logical operators.
Once is the identity logical operator which does nothing and is always found
at the start of the pipeline;
ScanAll is a logical operator which iteratively
produces all of the nodes in the graph; and
Produce is a logical operator
which takes data produced by another logical operator and produces data for the
query's result set.
A slightly more complicated example would be:
In this example, the
Filter logical operator is used to filter the matched
nodes because of the
WHERE n.prop = 42 construct. The
operator is used to find an edge between two nodes, in this case
which were matched previously using the
ScanAllByLabel logical operator (a
variant of the
ScanAll logical operator mentioned previously).
The execution of the query proceeds iteratively as follows. First, two vertices
:Node are found as the result of the two scans. Then, we try to find a
path that consists of the two vertices and an edge between them. If a path is
found, it is further filtered based on a property of one of the vertices.
Finally, if the path satisfied the filter, its two vertices are added to the
query's result set.
A simple example showcasing the fully general tree structure of the plan could be:
Merge logical operator (constructed as a result of the
can take input from up to 3 places. The
On Match and
On Create branches are
"pulled from" only if a match was found or if a new vertex has to be created,