ST_Curve
If the geometry-expression is of type ST_Curve, return the geometry-expression. If the geometry-expression is a geometry collection with a single element of type ST_Curve, return that element. 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 curve. The logic is equivalent to that used for CAST(geometry-expressionAS ST_Curve ).
If geometry-expression is already known to be an ST_Curve value, it is more efficient to use TREAT( geometry-expression AS ST_Curve ) than the ST_ToCurve method.