multiple INTERSECT support

[mweisgut]

TPC-DS queries contain multiple, not nested INTERSECT operations.

Example: TPC-DS query 38, netezza dialect:

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 1189 and 1189 + 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 1189 and 1189 + 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 1189 and 1189 + 11
) hot_cust
limit 100;