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[scan](improvement) Adjust parallel scanners num (apache#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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