Oracle 21c

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select sdo_geometry('linestring(1 2, 3 4, 5 6)').sdo_ordinates(1)
from   dual
ORA-00904: "MDSYS"."SDO_GEOMETRY"."SDO_ORDINATES": invalid identifier

select (sdo_geometry('linestring(1 2, 3 4, 5 6)').sdo_ordinates)(1)
from   dual
ORA-03001: unimplemented feature

select treat(
         st_geometry.from_wkt('Linestring(1 2, 3 4, 5 6)',26917)
         as st_linestring
       ).st_pointn(1)
from   dual
select text
from   all_source
where  type  = 'TYPE'
and    owner = 'MDSYS'
and    name  = 'ST_LINESTRING'
ORDER BY line

TEXT

TYPE ST_LINESTRING

UNDER ST_CURVE (

  CONSTRUCTOR FUNCTION ST_LINESTRING(apointarray ST_Point_Array)

               RETURN SELF AS RESULT,

  CONSTRUCTOR FUNCTION ST_LINESTRING(apointarray ST_Point_Array, asrid INTEGER)

 RETURN SELF AS RESULT,

  OVERRIDING MEMBER FUNCTION ST_IsSimple RETURN Integer DETERMINISTIC,

  MEMBER FUNCTION ST_Points(apointarray ST_Point_Array) RETURN ST_LineString)

select text
from   all_source
where  type  = 'TYPE'
and    owner = 'MDSYS'
and    name  = 'ST_CURVE'
ORDER BY line

TEXT

TYPE ST_CURVE

                                        UNDER ST_GEOMETRY (

  OVERRIDING MEMBER FUNCTION ST_Dimension RETURN Integer DETERMINISTIC,

  MEMBER FUNCTION ST_Points RETURN ST_Point_Array DETERMINISTIC,

  MEMBER FUNCTION ST_NumPoints RETURN INTEGER DETERMINISTIC,

  MEMBER FUNCTION ST_PointN(aposition INTEGER) RETURN ST_Point DETERMINISTIC,

  MEMBER FUNCTION ST_IsClosed RETURN Integer DETERMINISTIC,

  MEMBER FUNCTION ST_MidPointRep RETURN ST_Point_Array DETERMINISTIC,

  MEMBER FUNCTION ST_StartPoint RETURN ST_Point DETERMINISTIC,

  MEMBER FUNCTION ST_EndPoint RETURN ST_Point DETERMINISTIC,

  OVERRIDING MEMBER FUNCTION ST_IsSimple RETURN Integer DETERMINISTIC,

  MEMBER FUNCTION ST_IsRing RETURN Integer DETERMINISTIC,

  MEMBER FUNCTION ST_Length RETURN NUMBER DETERMINISTIC) NOT FINAL