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# ST_SharedPaths
Returns the paths shared by two lineal geometries, grouped by traversal direction. Both inputs must be a `LineString` or `MultiLineString` and must have the same SRID.
The result is a `GeometryCollection` containing exactly two `MultiLineString` elements:
* Element 0 contains paths traversed in the same direction by both inputs.
* Element 1 contains paths traversed in opposite directions.
Coordinates in both elements follow the direction of the first input, `A`. If the inputs share no path, including when they intersect only at points, both elements are empty.
## Signatures
```sql theme={"system"}
ST_SharedPaths(A: Geometry, B: Geometry)
```
## Parameters
The first `LineString` or `MultiLineString`, which determines the direction of returned paths.
The second `LineString` or `MultiLineString`, with the same SRID as `A`.
## Return type
A `GeometryCollection` of same-direction and opposite-direction paths, or `NULL` if either input is `NULL`.
Non-lineal inputs or mismatched SRIDs raise an error. The result retains the matching input SRID.
A non-empty result retains Z only when at least one input has Z and every returned coordinate resolves to a finite Z from the inputs. Otherwise, the entire result is two-dimensional. M values are always dropped, and a result with no shared paths is two-dimensional.
For inputs that traverse the same path more than once, direction follows the first matching source traversal. Swapping `A` and `B` can therefore change which element contains a shared path.
## Visual examples
### Same and opposite directions
Input `B` first follows `A`, leaves it, and later traverses another part of `A` backwards. The
single result therefore populates both direction buckets. Notice that the path in element 1 is
reoriented to follow `A`, even though `B` traverses it in the opposite direction.
### Multipart input and noded output
Input `A` can be a `MultiLineString`. In this example, `B` shares part of two different components
of `A`. The vertex at `(90 161)` belongs to `B` and splits the shared diagonal into two paths in
element 0.
### Point-only intersections
Crossing at an interior point and touching at an endpoint are both zero-dimensional contacts.
Neither is a shared path, so both `MultiLineString` elements are empty.
## Examples
### Same direction
Both inputs traverse the shared interval from left to right, so it appears in element 0.
```sql theme={"system"}
SELECT ST_AsText(ST_SharedPaths(
ST_GeomFromWKT('LINESTRING (0 0, 10 0)'),
ST_GeomFromWKT('LINESTRING (5 0, 15 0)')
));
```
```text theme={"system"}
GEOMETRYCOLLECTION (MULTILINESTRING ((5 0, 10 0)), MULTILINESTRING EMPTY)
```
### Opposite direction
The second input traverses the shared interval in reverse, so it appears in element 1. Its returned coordinates still follow `A`.
```sql theme={"system"}
SELECT ST_AsText(ST_SharedPaths(
ST_GeomFromWKT('LINESTRING (0 0, 10 0)'),
ST_GeomFromWKT('LINESTRING (15 0, 5 0)')
));
```
```text theme={"system"}
GEOMETRYCOLLECTION (MULTILINESTRING EMPTY, MULTILINESTRING ((5 0, 10 0)))
```
### Point-only intersection
Crossing at a point does not create a shared path.
```sql theme={"system"}
SELECT ST_AsText(ST_SharedPaths(
ST_GeomFromWKT('LINESTRING (0 0, 10 0)'),
ST_GeomFromWKT('LINESTRING (5 -5, 5 5)')
));
```
```text theme={"system"}
GEOMETRYCOLLECTION (MULTILINESTRING EMPTY, MULTILINESTRING EMPTY)
```