this is a bullshit, said this was meant to or will lead to not go index, to the union can walk index, but the actual use of time will have a variety of situations,

Oracle11g test

 create table my_big_table as the select level a, cast (' b '| | level as varchar2 (8)) b from dual connect by level & lt;=200000; 
The create index idx_my_big_table on my_big_table (a, b); 

The following two queries are equivalent, but will not go index

 select * 
The from my_big_table t1 
Where a t1. A=55654 
The or t1. B='b55654'; 

Select * 
The from my_big_table t1 
Where a t1. A=55654 
The union 
Select * 
The from my_big_table t1 
Where a t1. B='b55654 

Try again in a proper index

 create table my_big_table2 as the select level a, cast (' b '| | level as varchar2 (8)) b from dual connect by level & lt;=200000; 
The create index idx_my_big_table2_1 on my_big_table2 (a); 
The create index idx_my_big_table2_2 on my_big_table2 (b); 

And you can see the optimizer self-assertion give turned bitmap,

With a hint for compulsory modify execution plan

Can be seen from the execution plan does to the union more efficient, but the optimizer will automatically help you to do this step, and it is according to the statistics do intelligent judgment, no manual to the union,

Another group of data of the exotic

 create table my_big_table3 as select mod (level 2), a, cast (' b '| | level as varchar2 (8)) b from dual connect by level & lt;=200000; 
The create index idx_my_big_table3_1 on my_big_table3 (a); 
The create index idx_my_big_table3_2 on my_big_table3 (b); 

Select * from my_big_table3 t1 where t1. A=1 or t1. B='b1235'; Or just take t1. A=1 this condition, you can't do it again walk index,

Again with my work just write a SQL for example

 
Select * 
The from aaa. Tab1 t1 
Where a t1. Aaaa_date='20200504' 
And t1. Day_id=substr (' 20200504 ', 7, 2) 
And t1. Aaaa_no='01' 

And (
(# {levelType}='1') 
Or (# {levelType}='2' and # {saleId}=t1, col1) 
Or (# {levelType}='3' and # {saleId}=t1. Col2) 
) 

Aaa. Tab1 a partition table despite 40 w data, but I was to a full table query, why do you want to rewrite? Even if I only find a few data will not be changed, because there is no index on this table, if no index, the above all can not stand

 
Select * 
The from my_big_table t1 
Where a t1. A=55654 
The or t1. B='b55654'; 

Select * 
The from my_big_table t1 
Where a t1. A=55654 
The union 
Select * 
The from my_big_table t1 
Where a t1. B='b55654'