Optimizing Queries in MySQL - mysql

Question

Is Query 1 more optimized say for example for a larger database than Query 2 even by slight or am I just doubling the work with an additional WHERE clause?
Query 1:
SELECT sample_data
FROM table1 INNER JOIN table2 ON table1.key = table2.key
WHERE table1.key = table2.key;
Query 2:
SELECT sample_data
FROM table1 INNER JOIN table2 ON table1.key = table2.key;
Because I read this article saying that using filters in JOIN clauses improve the performance..:

Answer 1

Is Query 1 more optimized say for example for a larger database than Query 2?
No, it is not more optimized. Query 2 is the correct way to handle the JOIN. Query 1 does the same thing, but with extra verbiage for the MySQL server software to scrub out as it figures out how to satisfy your query.
The advice at the Adobe documentation about filtering both tables in a join does not relate to the join's ON-condition. Their example says to do this...
SELECT whatever, whatever
FROM table1
JOIN table2 ON table2.table1_id = table1.table1_id
WHERE table1.date >= '2021-01-01'
AND table2.date >= '2021-01-01' /* THIS LINE IS WHAT THEY SUGGEST */
Their suggestion, from 2015, has to do with filtering non-join attributes from both tables. It's a suggestion to use to optimize a query if it just isn't fast enough for you. And, in my experience, it's not a very good suggestion. Ignore it, at least for now. More recent MySQL versions have gotten more efficient.
Let me add to this. SQL is a so-called "declarative" language. You declare what you want and the MySQL server figures out how to get it for you. SQL software is getting really good at doing that; keep in mind that MySQL is now a quarter century old. In that time its programmers have been continuously making it smarter at figuring out how to get stuff. You probably can't outsmart it. But you may need to add indexes when your tables get really big. https://use-the-index-luke.com/
Other languages are "procedural": you, as a programmer, spell out a procedure for getting what you want. You don't need to do that for SQL.

Answer 2

I like to put it this way:
ON is where you specify how the tables are related.
WHERE is for filtering.
That makes it easy for a human reading the query to understand it.
In reality (for MySQL), JOIN (aka INNER JOIN) treats ON and WHERE identically. That is, there is no performance difference. Your Query 1 unnecessarily specifies the "relation" twice.
Also, MySQL's Optimizer is smart enough to realize when two columns have the same value. For example,
SELECT ...
FROM a
JOIN bb ON a.foo = bb.foo
WHERE a.foo = 123
If the Optimizer decides that starting with the filter bb.foo = 123 is more optimal, it will do so. Note: This is not the same as the example you showed; it joins on one thing (id) but filters on another (date). The two queries there are not equivalent!
LEFT JOIN, necessarily treats ON and WHERE differently. (But that is another topic.)