ST_MultiCurve
If the geometry-expression is of type ST_MultiCurve, returns the geometry-expression. If the geometry-expression is a geometry collection containing only curves, returns a multicurve object containing the elements of the geometry-expression. If the geometry-expression is of type ST_Curve then return a multicurve value containing one element, the geometry-expression. If the geometry-expression is the empty set, returns an empty set of type ST_MultiCurve. Otherwise, raises an exception condition.
The spatial reference system identifier of the result is the same as the spatial reference system of the geometry-expression.
If geometry-expression is already known to be an ST_MultiCurve value, it is more efficient to use TREAT( geometry-expression AS ST_MultiCurve ) than the ST_ToMultiCurve method.
Note
By default, ST_ToMultiCurve uses the original format for a geometry, if it is available. Otherwise, the internal format is
used. For more information about internal and original formats, see STORAGE FORMAT clause, CREATE SPATIAL REFERENCE SYSTEM statement.