ST_CircularString
If the geometry-expression is of type ST_CircularString, return the geometry-expression. If the geometry-expression is of type ST_CompoundCurve with a single element which is of type ST_CircularString, return that element. If the geometry-expression is a geometry collection with a single element of type ST_CircularString, return that element. If the geometry-expression is the empty set, return an empty set of type ST_CircularString. 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.
Convert this geometry to a circularstring. The logic is equivalent to that used for CAST( geometry-expression AS ST_CircularString ).
If geometry-expression is already known to be an ST_CircularString value, it is more efficient to use TREAT( geometry-expression AS ST_CircularString ) than the ST_ToCircular method.