Please can someone explain to me, again, why

select Max(MYCOL) from MYTAB

doesn't use the primary key

PLAN (MYTAB NATURAL)

Current memory = 3074072
Delta memory = -20
Max memory = 3217516
Elapsed time= 0.27 sec
Buffers = 150
Reads = 1811
Writes 0
Fetches = 894779

of which it is the first field

CONSTRAINT PK_MYTABPRIMARY KEY (MYCOL, OTHERCOL)

and instead reads thousands of pages from disk and takes over a quarter
of a second?

(I'm just curious and wanting to understand, really. I'm going to fix
the actual problem with a change of approach, that's needed for other
reasons anyway, which eliminates the problem query.)

--
Tim Ward