Query with ST_BUFFER gives different results and is very slow compared with MySQL

SELECT ST_BUFFER(

    ST_UNION(

        MULTILINESTRINGFROMTEXT( ' MULTILINESTRING( ( 3 4 , 2 5 , 7 6 , 1 8 ) , ( 0 0 , 1 6 , 0 1 , 8 9 , 2 4 , 6 1 , 3 5 , 4 8 ) , ( 9 3 , 5 4 , 1 8 , 4 2 , 5 8 , 3 0 ) )' ) ,

        MULTILINESTRINGFROMTEXT( ' MULTILINESTRING( ( 3 4 , 3 1 , 2 7 , 4 2 , 6 2 , 1 5 ) ) ' )

    ), 

    AREA(

        MULTIPOLYGONFROMTEXT( ' MULTIPOLYGON( ( ( 7 7 , 6 7, 1 5, 7 6 , 7 7 ) ) , ( (0 5, 3 5, 3 4, 1 4 , 1 3 , 3 3 , 3 0 , 0 0 , 0 5), ( 1 1 , 2 1 , 2 2 , 1 2 , 1 1 ) ) ) ' )

    )

);