index usage in various scenarious

Hello,

QUESTION1: Can somebody clarify in what of the proposed scenarios is the
following index used? Any further comment will be greatly appreciated.

QUESTION2: Does any other scenarios when an index is NOT used (and someone
might possibly expect it is used) come to your mind? Thank you very much for
your time and wish you a happy day:-)

CREATE INDEX idx_margincall_bussdt ON margincall USING btree (businessdate);

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SCENARIO 1:

select MC.IDTRADINGACCOUNT

from MARGINCALL MC

join DEFAULTINGACCOUNT DA on DA.BUSINESSDATE = MC.BUSINESSDATE
and DA.IDTRADINGACCOUNT = MC.IDTRADINGACCOUNT

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SCENARIO 2:

select MC.IDTRADINGACCOUNT

from MARGINCALL MC

where MC.BUSINESSDATE = '17.11.2008';

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SCENARIO 3:

select MC.IDTRADINGACCOUNT

from MARGINCALL MC

join DEFAULTINGACCOUNT DA on DA.BUSINESSDATE = MC.BUSINESSDATE
and DA.IDTRADINGACCOUNT = MC.IDTRADINGACCOUNT

where MC.BUSINESSDATE = '17.11.2008';

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SCENARIO 4 + QUESTION:

Is usage of indexes during execution of queries affected by db-links?

E.g.:

select MC.IDTRADINGACCOUNT

from MARGINCALL(at)DB_LINK_NAME MC

where MC.BUSINESSDATE = '17.11.2008';

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Adam Slachta