[PR] [scan](improvement) Adjust parallel scanners num [doris]

[via GitHub]

HappenLee opened a new pull request, #38430:
URL: https://github.com/apache/doris/pull/38430

   ## Proposed changes
   
   Before：
   ```
   mysql [tpcds]>select  count(*) from (     select distinct c_last_name, 
c_first_name, d_date     from store_sales, date_dim, customer           where 
store_sales.ss_sold_date_sk = date_dim.d_date_sk       and 
store_sales.ss_customer_sk = customer.c_customer_sk       and d_month_seq 
between 1186 and 1186 + 11   intersect     select distinct c_last_name, 
c_first_name, d_date     from catalog_sales, date_dim, customer           where 
catalog_sales.cs_sold_date_sk = date_dim.d_date_sk       and 
catalog_sales.cs_bill_customer_sk = customer.c_customer_sk       and 
d_month_seq between 1186 and 1186 + 11   intersect     select distinct 
c_last_name, c_first_name, d_date     from web_sales, date_dim, customer        
   where web_sales.ws_sold_date_sk = date_dim.d_date_sk       and 
web_sales.ws_bill_customer_sk = customer.c_customer_sk       and d_month_seq 
between 1186 and 1186 + 11 ) hot_cust limit 100;
   +----------+
   | count(*) |
   +----------+
   | 24874049 |
   +----------+
   1 row in set (53 sec)
   ```
   
   After:
   ```
   mysql [tpcds]>select  count(*) from (     select distinct c_last_name, 
c_first_name, d_date     from store_sales, date_dim, customer           where 
store_sales.ss_sold_date_sk = date_dim.d_date_sk       and 
store_sales.ss_customer_sk = customer.c_customer_sk       and d_month_seq 
between 1186 and 1186 + 11   intersect     select distinct c_last_name, 
c_first_name, d_date     from catalog_sales, date_dim, customer           where 
catalog_sales.cs_sold_date_sk = date_dim.d_date_sk       and 
catalog_sales.cs_bill_customer_sk = customer.c_customer_sk       and 
d_month_seq between 1186 and 1186 + 11   intersect     select distinct 
c_last_name, c_first_name, d_date     from web_sales, date_dim, customer        
   where web_sales.ws_sold_date_sk = date_dim.d_date_sk       and 
web_sales.ws_bill_customer_sk = customer.c_customer_sk       and d_month_seq 
between 1186 and 1186 + 11 ) hot_cust limit 100;
   +----------+
   | count(*) |
   +----------+
   | 24874049 |
   +----------+
   1 row in set (40.45 sec)
   ```
   
   <!--Describe your changes.-->
   
   


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