# meta

> **Info**
>
> This module got deprecated in `v2.21.0` with the addition of the [`SHOW SCHEMA INFO`](https://memgraph.com/docs/querying/schema) feature.

The **meta** module provides a set of procedures for generating metadata about the database.

| Trait               | Value      |
| ------------------- | ---------- |
| **Module type**     | util       |
| **Implementation**  | C++        |
| **Parallelism**     | parallel   |

## Procedures

### `stats_online()`

The procedure retrieves the graph metadata in **O(1)** complexity. In order for
it to work correctly, you need to set up a trigger by running the following
query:

```cypher
CREATE TRIGGER meta_trigger BEFORE COMMIT 
EXECUTE CALL meta.update(createdObjects, deletedObjects, removedVertexProperties, removedEdgeProperties, setVertexLabels, removedVertexLabels);
```

The `stats_online` procedure tracks the created, deleted or modified data after
the trigger is added. If you want to return the metadata about the whole graph
you need to run the  procedure with the `update_stats` flag set to `true`, but
only **once**. That flag will cause the procedure to traverse the whole graph to
update the metadata. After that run, you can run the procedure with the `update_stats`
flag set to `false` and the procedure will return the metadata in **O(1)**
complexity.

#### Input:

- `subgraph: Graph` (**OPTIONAL**) ➡ A specific subgraph, which is an [object of type Graph](https://memgraph.com/docs/advanced-algorithms/run-algorithms#run-procedures-on-subgraph) returned by the `project()` function, on which the algorithm is run. 
If subgraph is not specified, the algorithm is computed on the entire graph by default.
- `update_stats: bool (default=false)` ➡ If true, the procedure traverses the
  whole graph to update the metadata. Otherwise, it returns the stored metadata.

#### Output:

- `labelCount: int` ➡ The number of unique labels in nodes.
- `relationshipTypeCount: int` ➡ The number of unique relationship types (labels).
- `nodeCount: int` ➡ The number of nodes in the graph.
- `relationshipCount: int` ➡ The number of relationships in the graph.
- `labels: Map[string: int]` ➡ A map with the following (key, value) pairs:
  - `label : number_of_occurrences`
- `relationshipTypes: Map[string: int]` ➡ A map with the following (key, value) pairs:
  - `(:label)-[:relationship_type]->() : number_of_occurrences`
  - `()-[:relationship_type]->(:label) : number_of_occurrences`
  - `()-[:relationship_type]->() : number_of_occurrences`
- `relationshipTypesCount: Map[string: int]` ➡ A map with the following (key, value) pairs:
  - `relationship_type : number_of_occurrences`
- `stats` ➡ A map which contains all of the above.

#### Usage:

Create the following objects in the database:

```cypher
MERGE (a:Node {id: 0}) MERGE (b:Node {id: 1}) CREATE (a)-[:Relation1]->(b);
MERGE (a:Node {id: 1}) MERGE (b:Node {id: 2}) CREATE (a)-[:Relation1]->(b);
MERGE (a:Node {id: 2}) MERGE (b:Node {id: 0}) CREATE (a)-[:Relation1]->(b);
MERGE (a:Node {id: 3}) MERGE (b:Node {id: 3}) CREATE (a)-[:Relation2]->(b);
MERGE (a:Node {id: 3}) MERGE (b:Node {id: 4}) CREATE (a)-[:Relation2]->(b);
MERGE (a:Node {id: 3}) MERGE (b:Node {id: 5}) CREATE (a)-[:Relation2]->(b);
```

Run stats using the following query:

```cypher
CALL meta.stats_online() YIELD stats;
```

```plaintext
+-------------------------------------------------------+
|     stats                                             |
+-------------------------------------------------------+
|                                                       |
|{                                                      |
|   "labelCount": 1,                                    |
|   "labels": {                                         |
|      "Node": 6                                        |
|   },                                                  |
|   "nodeCount": 6,                                     |
|   "propertyKeyCount": 1,                              |
|   "relationshipCount": 6,                             |
|   "relationshipTypeCount": 2,                         |
|   "relationshipTypes": {                              |
|      "()-[:Relation1]->()": 3,                        |
|      "()-[:Relation1]->(:Node)": 3,                   |
|      "()-[:Relation2]->()": 3,                        |
|      "()-[:Relation2]->(:Node)": 3,                   |
|      "(:Node)-[:Relation1]->()": 3,                   |
|      "(:Node)-[:Relation2]->()": 3                    |
|   },                                                  |
|   "relationshipTypesCount": {                         |
|      "Relation1": 3,                                  |
|      "Relation2": 3                                   |
|   }                                                   |
|}                                                      |
|                                                       |
+-------------------------------------------------------+
```

### `stats_offline()`

Retrieves graph metadata by traversing the whole graph. The `stats_online`
procedure should be preferred because of the better complexity unless you don't
want to use triggers.

#### Input:

- `subgraph: Graph` (**OPTIONAL**) ➡ A specific subgraph, which is an [object of type Graph](https://memgraph.com/docs/advanced-algorithms/run-algorithms#run-procedures-on-subgraph) returned by the `project()` function, on which the algorithm is run. 
If subgraph is not specified, the algorithm is computed on the entire graph by default.

#### Output:

- `labelCount: int` ➡ The number of unique labels in nodes.
- `relationshipTypeCount: int` ➡ The number of unique relationship types (labels).
- `nodeCount: int` ➡ The number of nodes in the graph.
- `relationshipCount: int` ➡ The number of relationships in the graph.
- `labels: Map[string: int]` ➡ A map with the following (key, value) pairs:
  - `label : number_of_occurrences`
- `relationshipTypes: Map[string: int]` ➡ A map with the following (key, value) pairs:
  - `(:label)-[:relationship_type]->() : number_of_occurrences`
  - `()-[:relationship_type]->(:label) : number_of_occurrences`
  - `()-[:relationship_type]->() : number_of_occurrences`
- `relationshipTypesCount: Map[string: int]` ➡ A map with the following (key, value) pairs:
  - `relationship_type : number_of_occurrences`
- `stats` ➡ A map which contains all of the above.

#### Usage:

Create the following objects in the database:

```cypher
MERGE (a:Node {id: 0}) MERGE (b:Node {id: 1}) CREATE (a)-[:Relation1]->(b);
MERGE (a:Node {id: 1}) MERGE (b:Node {id: 2}) CREATE (a)-[:Relation1]->(b);
MERGE (a:Node {id: 2}) MERGE (b:Node {id: 0}) CREATE (a)-[:Relation1]->(b);
MERGE (a:Node {id: 3}) MERGE (b:Node {id: 3}) CREATE (a)-[:Relation2]->(b);
MERGE (a:Node {id: 3}) MERGE (b:Node {id: 4}) CREATE (a)-[:Relation2]->(b);
MERGE (a:Node {id: 3}) MERGE (b:Node {id: 5}) CREATE (a)-[:Relation2]->(b);
```
Run stats using the following query:

```cypher
CALL meta.stats_offline() YIELD stats;
```

```plaintext
+-------------------------------------------------------+
|     stats                                             |
+-------------------------------------------------------+
|                                                       |
|{                                                      |
|   "labelCount": 1,                                    |
|   "labels": {                                         |
|      "Node": 6                                        |
|   },                                                  |
|   "nodeCount": 6,                                     |
|   "propertyKeyCount": 1,                              |
|   "relationshipCount": 6,                             |
|   "relationshipTypeCount": 2,                         |
|   "relationshipTypes": {                              |
|      "()-[:Relation1]->()": 3,                        |
|      "()-[:Relation1]->(:Node)": 3,                   |
|      "()-[:Relation2]->()": 3,                        |
|      "()-[:Relation2]->(:Node)": 3,                   |
|      "(:Node)-[:Relation1]->()": 3,                   |
|      "(:Node)-[:Relation2]->()": 3                    |
|   },                                                  |
|   "relationshipTypesCount": {                         |
|      "Relation1": 3,                                  |
|      "Relation2": 3                                   |
|   }                                                   |
|}                                                      |
|                                                       |
+-------------------------------------------------------+
```

### `update()`

Helper function used in the trigger for the `stats_online` procedure. You should not use this manually.

### `reset()`

Deletes all stored metadata about the graph.

#### Input:

None.

#### Output:

None.

#### Usage:

```cypher
CALL meta.reset();
```