jiayuasu opened a new issue, #2709:
URL: https://github.com/apache/sedona/issues/2709
## Expected behavior
`ST_LineMerge` should return a `LineString` unchanged when given a single
`LineString` as input, consistent with PostGIS and GEOS (`GEOSLineMerge`).
A single `LineString` is already "merged" — there is nothing to merge.
### PostGIS / GEOS behavior
```sql
SELECT ST_AsText(ST_LineMerge('LINESTRING(0 0, 1 1)'));
-- Returns: LINESTRING(0 0, 1 1)
```
```python
>>> from shapely import line_merge
>>> from shapely.geometry import LineString
>>> line_merge(LineString([(0, 0), (1, 1)]))
<LINESTRING (0 0, 1 1)>
```
## Actual behavior
Sedona returns `GEOMETRYCOLLECTION EMPTY` for `LineString` input:
```python
>>> df.selectExpr("ST_LineMerge(ST_GeomFromText('LINESTRING(0 0, 1
1)'))").show()
# Returns: GEOMETRYCOLLECTION EMPTY
```
## Root cause
In [`Functions.java` line
1198](https://github.com/apache/sedona/blob/master/common/src/main/java/org/apache/sedona/common/Functions.java#L1198),
`lineMerge` only handles `MultiLineString` and falls through to returning an
empty `GeometryCollection` for all other types — including `LineString`:
```java
public static Geometry lineMerge(Geometry geometry) {
if (geometry instanceof MultiLineString) {
// ... merge logic ...
}
return geometry.getFactory().createGeometryCollection(); // <--
LineString hits this
}
```
## Suggested fix
Add a check for `LineString` before the `MultiLineString` block:
```java
public static Geometry lineMerge(Geometry geometry) {
if (geometry instanceof LineString) {
return geometry; // A single LineString is already merged
}
if (geometry instanceof MultiLineString) {
// ... existing merge logic ...
}
return geometry.getFactory().createGeometryCollection();
}
```
## Environment
- Sedona version: 1.9.0-SNAPSHOT (master)
- Found while implementing geopandas `line_merge()` wrapper (#2230, #2701)
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