EXPLAIN EXTENDED

Answering questions asked on the site.

Magerio asks:

Hello, thanks for your great post about advanced row sampling, it helped me a lot.

I have a question.

I am using your SQL query to show a portfolio of products and these products have a certain integer quality level (the lower, the better).

What if I want to show the best 3 products of each category?

Just a quick reminder: the article mentioned describes a query to emulate ROW_NUMBER() in MySQL. Since MySQL does not support analytic (window) functions natively, we had to do some kind of a trick to mimic their behaviour.

The original article described a query to obtain 15 first id's for each grouper. The idea behind this query was quite simple:

1. Take a list of groupers using DISTINCT
2. For each grouper, find the 15th id using LIMIT in a correlated subquery. We have now a set of (grouper, id15) pairs: for any given grouper we have the id of the 15th row (ordered by id).
3. Join this resultset with the main table so that for each grouper, the id's less or equal to the 15th id are selected. We then will have 15 lowest id's for each grouper that is exactly what we were after.

This emulated ROW_NUMBER() OVER (PARTITION BY grouper ORDER BY id): a function with a single PARTITION BY expression and a single ORDER BY expression.

But in your case there are multiple ORDER BY expressions. It may be not so obvious (since the question mentions only one ordering criterion, namely, quality). However, the solution above relied on the fact that the field in ORDER BY was unique, so we now that there are exactly 15 rows with id less or equal to the id returned with LIMIT 15. Since this is most probably not the case with quality, we should include id into the ORDER BY clause so that it makes the ordering conidition unique.

A subquery in MySQL, however, can only return a single value.

To work around this, we should use two subqueries that will return us a single column in an ORDER BY condition.

Let's create a sample table:
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