CREATE TABLE TEST
(
ID1 INTEGER,
ID2 INTEGER,
ID3 INTEGER,
X NUMERIC(18,2),
CONSTRAINT PK_TEST PRIMARY KEY(ID1, ID2, ID3)
);


CREATE INDEX IXA_TEST__X ON TEST(X);
CREATE INDEX IXA_TEST__ID1_X ON TEST(ID1, X);


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SELECT * FROM TEST T WHERE T.ID1=1 AND T.X>0

PLAN (T INDEX (IXA_TEST__ID1_X))

index IXA_TEST__ID1_X is used
--------------------------------------------------------------------------------


SELECT * FROM TEST T WHERE T.ID1=1 AND T.X>0 ORDER BY T.ID1, T.ID2, T.ID3

PLAN (T ORDER PK_TEST INDEX (IXA_TEST__X))

index IXA_TEST__X - suboptimal
--------------------------------------------------------------------------------

as you can see adding ORDER BY which consume some index (PK_TEST)
cause suboptimal choice of index (IXA_TEST__X)

--------------------------------------------------------------------------------

if query is changed to order by not by index
SELECT * FROM TEST T WHERE T.ID1=1 AND T.X>0 ORDER BY T.ID1+0, T.ID2, T.ID3

PLAN SORT (T INDEX (IXA_TEST__ID1_X))


It can be releated to CORE-5795 but this can be different case