Can I get an explanation why an index isn't used for the second query?
Should I incorporate tc.R in to an index (its selectivity is
questionable, see below).

The table alias 'tc' is from the query that contains this EXISTS clause.

-- uses index on d.G and dp.N
EXISTS (
SELECT 1
FROM TAB1 dp
JOIN TAB2 d ON d.P=dp.N
WHERE dp.A='Y' AND d.G STARTING WITH SUBSTRING(tc.T FROM 1 FOR 2)
)

-- only uses index on dp.N, d is natural
EXISTS (
SELECT 1
FROM TAB1 dp
JOIN TAB2 d ON d.P=dp.N
WHERE dp.A='Y' AND
((tc.R=1 AND d.G STARTING WITH SUBSTRING(tc.TCC FROM 1 FOR 2)) OR
(tc.R=2 AND d.G STARTING WITH SUBSTRING(tc.TCC FROM 1 FOR 4)) OR
(tc.R=3 AND d.G STARTING WITH SUBSTRING(tc.TCC FROM 1 FOR 6)) OR
(tc.R=4 AND d.G STARTING WITH tc.TCC))
)

Tc.R distribution of values

R-VAL COUNT
-----------
1 94
2 601
3 825
4 5448

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