DECLARE
c_Count NUMBER;
BEGIN
SELECT COUNT(*) INTO c_Count
FROM DBA_INDEXES
WHERE TABLE_NAME = 'SOME_TABLE_NAME'
AND INDEX_NAME = 'SOME_INDEX_NAME'
AND OWNER = 'SOME_OWNER'
AND TABLE_OWNER = 'SOME_OWNER';
IF c_Count = 0 THEN
execute immediate 'CREATE INDEX SOME_OWNER.SOME_INDEX ON SOME_OWNER.SOME_TABLE (SOME_KEY)';
END IF;
END;