MySQL – run SELECT statement on another server without defining the table structure

A few quick ideas:

The local server initiates a connection to the remote server, runs a query and fetches appropriate rows from the remote server. This all takes time. Worse, if you trace such activity with SQL Profiler, you may find that rows are fetched from the remote server to the local server one at a time, or in very small batches. This is very inefficient, and the more rows it fetches the worse and worse it gets.

If all data is local, none of that happens and the data can be retrieved much more quickly, especially if it is sitting in the data cache.

Even though there may be indexing on tables on the remote server, SQL may not be able to take advantage of them while it can build a local query plan that does take advantages of indexing.

For very large remote result sets (in terms of data size), you may run into issues where SQL needs to grow data or log files to accommodate the data. 7 seconds probably doesn't qualify, since it probably isn't bringing back much data.

Lastly, if the remote query is to a production system and the local query is to a dev or test system, load on the production server (which does not exist on the dev server) may cause blocking of the remote query. This would also slow down the observed performance of the remote query.

Based on your current code, it appears that a product can have at most one match in either customer_price_increment or general_price_increment. With that fact in mind, I would probably try a different approach.

First, I would rewrite the base query returning all the prices like this:

SELECT
  p.id AS product_number,
  p.name AS product_name,
  p.redemption_price,
  COALESCE(p.redemption_price * cpi.increment_percentage / 100, 0) AS customer_price_increment,
  COALESCE(p.redemption_price * gpi.increment_percentage / 100, 0) AS general_price_increment,
  p.redemption_price * (1 + COALESCE(cpi.increment_percentage, gpi.increment_percentage, 0) / 100) AS sale_price
FROM product AS p
LEFT JOIN customer_price_increment AS cpi ON cpi.id = p.id AND cpi.customer_id = 4
LEFT JOIN general_price_increment AS gpi ON gpi.id = p.id

To filter on sale_price, I would just use the above query as a derived table, so that I could reference the sale_price alias and avoid repeating the entire expression in the WHERE clause:

SELECT *
FROM (
    SELECT
      p.id AS product_number,
      p.name AS product_name,
      p.redemption_price,
      COALESCE(p.redemption_price * cpi.increment_percentage / 100, 0) AS customer_price_increment,
      COALESCE(p.redemption_price * gpi.increment_percentage / 100, 0) AS general_price_increment,
      p.redemption_price * (1 + COALESCE(cpi.increment_percentage, gpi.increment_percentage, 0) / 100) AS sale_price
    FROM product AS p
    LEFT JOIN customer_price_increment AS cpi ON cpi.id = p.id AND cpi.customer_id = 4
    LEFT JOIN general_price_increment AS gpi ON gpi.id = p.id
) AS s
WHERE sale_price > 250
;

I believe you could also make the first query a view and then just select from it filtering the results as necessary:

SELECT *
FROM sale_prices_view
WHERE sale_price > 250
;