If the geometry-expression is of type ST_Polygon, returns the geometry-expression. If the geometry-expression is of type ST_CurvePolygon, returns geometry-expression.ST_CurvePolyToPoly(). If the geometry-expression is a geometry collection with a single element of type ST_CurvePolygon, returns that element. If the geometry-expression is the empty set, returns an empty set of type ST_Polygon. 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.
Convert the geometry to a polygon. The logic is equivalent to that used for CAST(geometry-expressionAS ST_Polygon ). If the geometry-expression is a curve polygon, it is interpolated using ST_CurvePolyToPoly().
If geometry-expression is already known to be an ST_Polygon value, it is more efficient to use TREAT( geometry-expression AS ST_Polygon ) than the ST_ToPolygon method.