【数据库负载过高问题】字段类型转换导致查询不能使用索引

1. 原SQL语句

SELECT TITLE, CODE, TYPE, YEAR, SS_DATE,URL 
    FROM (SELECT TITLE, CODE, TYPE,TO_CHAR(SS_DATE,'yyyy-mm-dd') SSBDATE,URL
          FROM TABLE_A
        WHERE 1=1 AND ISVALIDATE='1'
        UNION ALL
        SELECT TITLE,CODE,YEAR,SS_DATE,UBRL
            FROM TABLE_B
        WHERE 1=1 AND ISVALIDATE='1' )
     WHERE 1=1
        AND SS_DATE>='2019-07-09'
        AND SS_DATE<='2019-07-10'
     ORDER BY SS_DATE; 

查询10条数据，执行计划查看要1.4秒以上。

2 改下后SQL语句：将to_char字段类型转换改成子查询外进行。

SELECT TITLE, CODE, TYPE, YEAR, SS_DATE,URL 
    FROM (SELECT TITLE, CODE, TYPE,SS_DATE,URL
          FROM TABLE_A
        WHERE 1=1 AND ISVALIDATE='1'
        UNION ALL
        SELECT TITLE,CODE,YEAR,SS_DATE,UBRL
            FROM TABLE_B
        WHERE 1=1 AND ISVALIDATE='1' )
     WHERE 1=1
        AND SS_DATE>=TO_DATE('2019-07-09','YYYY-MM-DD')
        AND SS_DATE<=TO_DATE('2019-07-10','YYYY-MM-DD')
     ORDER BY SS_DATE DESC; 

查询计划发现，同样数据需要0.02秒。