Stan,

d> If I set up a PK to consist of the first two (or three or...) columns
d> of a table, is each column individually indexed?

No, you get an unique multisegment index.

d> That is, if I
d> create a query that has a WHERE condition on, say, the second column,
d> is that column individually indexed (because it is part of the PK),
d> or does my query lack indexing assistance?

It lacks. So choose the sequence of the fields in the key with care.
Someone else might argue that multicolumn keys should not
be used anyway, but I prefer to stay away from that right now. :-)

Ciao
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