Covering indices that span two tables

kylie :

Is it possible to create covering indices that span across tables? Suppose I want to optimize this query:

SELECT tbl1.col1,
       tbl2.col2
  FROM tbl1 INNER JOIN
       tbl2 ON tbl1.col1 = tbl2.col1
 WHERE tbl1.col3 = ... 

Will creating these indices help?

INDEX (col3, col1) -- tbl1's
INDEX (col1, col2) -- tbl2's 

Rick James :

Assuming this is the complete query...

SELECT  a.col1, b.col2
    FROM  tbl1 AS a
    INNER JOIN  tbl2 AS b  ON a.col1 = b.col1
    WHERE  a.col3 = constant

Analysis:

Because of the Where clause, tbl1 will be looked at first and this will be the optimal index:

INDEX(col3, col1) == handles the WHERE, plus it is "covering"

Then the Nested Loop Join into tbl2 will need an index starting with col:

INDEX(col1, col2) -- again "covering"

See also http://mysql.rjweb.org/doc.php/index_cookbook_mysql

The title ("... span two tables") is deceptive. Neither of these indexes involves more than one table. INDEX(col2, col3) is not possible because it involves more than one table.