If the geometry-expression is of type ST_LineString, return the geometry-expression. If the geometry-expression is of type ST_CircularString or ST_CompoundCurve, return geometry-expression.ST_CurveToLine(). If the geometry-expression is a geometry collection with a single element of type ST_Curve, return that element cast as ST_LineString. If the geometry-expression is the empty set, return an empty set of type ST_LineString. Otherwise, raise an exception condition.
The spatial reference system identifier of the result is the same as the spatial reference system of the geometry-expression.
Converts the geometry to a linestring. The logic is equivalent to that used for CAST( geometry-expression AS ST_LineString ). If the geometry-expression is a circularstring or compound curve, it is interpolated using ST_CurveToLine().
If geometry-expression is already known to be an ST_LineString value, it is more efficient to use TREAT( geometry-expression AS ST_LineString ) than the ST_ToLineString method.