索引文本比较

Question

SELECT
                    sob_datas.id,
                    sob_datas.sob_field_name
                FROM sob_datas
                WHERE sob_form_id = '.$id.' AND
                EXISTS(
                    SELECT
                        sob_datas_to_links.id
                    FROM sob_datas_to_links
                    INNER JOIN sob_datas AS sub_sob_datas ON 
                        sub_sob_datas.id = sob_datas_to_links.id
                    WHERE sob_datas_id = sob_datas.id AND sub_sob_datas.sob_field_value != sob_datas.sob_field_value
                )

我的代码显示WHERE sob_datas_id = sob_datas.id AND sub_sob_datas.sob_field_value != sob_datas.sob_field_value比较str1 != str2 ....对于这种搜索，索引/优化的最佳方式是什么？

sob_field_value =字段类型：text