SQL way to index across tables? - mysql

Question

Is there a way to create a multi column index across tables?
For example, if I had the following tables:
Foo (Table Name)
FooID (PK)
BarID (FK)
FooName
Bar (Table Name)
BarID (PK)
BarName
I can do a
SELECT *
FROM Foo
LEFT JOIN Bar ON Foo.BarID = Bar.BarID
WHERE
FooName < "Bob"
AND BarName > "Smith";
In this case, I want a multi column index against Foo.FooName then Bar.BarName.
I did some research but wasn't able to find anything, perhaps I'm not using the right terms. My question may depend on the SQL engine, in which case I'm interested in MySQL specifically, but I am interested in any other engines as well.
Doing the multi column index on Foo with the Foreign Key doesn't help, as the underlying value of its Name is what I want for the speed.

Answer 1

Came across my own post years later and figured I could add some details, in case others have similar issues. As Mark B pointed out, each index is per table, however we can set things up that make this efficent, see below.
There are a couple of different things going on here, so can use indexes to help accomplish what we need. We need an index to help filter the main table, then an index that works well for the join and filter of the 2nd table. To help accomplish this, we can create the following 2 indexes:
CREATE INDEX idx_fooname ON Foo (FooName);
CREATE INDEX idx_barid_barname ON Bar (BarID, BarName);
Once those indexes are in place, a query can be used like:
SELECT *
FROM Foo USE INDEX(idx_fooname)
LEFT JOIN Bar USE INDEX (idx_barid_barname) ON Foo.BarID = Bar.BarID
WHERE
FooName < "Bob"
AND BarName > "Smith";

Answer 2

Smells like "over-normalization". Might it be worth moving those two fields into the same table?
Akiban was an Engine that could do cross-table JOINs, etc. But it no longer exists.
"Materialized Views" do not exist in MySQL (unless you implement them yourself).

Answer 3

As xQbert mentioned, you can use materialized views (or indexed views if you use Microsft SQL Server).
But first you have to change your LEFT JOIN to a INNER JOIN (because of materialized/indexed views can't handle outer joins. The left join does not seem to make sense in your query because of the WHERE ... BarName > "Smith".
That link might help you if you use SQL Server: https://www.simple-talk.com/sql/learn-sql-server/sql-server-indexed-views-the-basics/
After you created a materialized/indexed view, you can query the view directly (i'm not sure if the query optimizer will use it automatically).
Be aware that the materialized/indexed view will reduce your performance when you INSERT, DELETE or UPDATE into the used tables (what every index will do, too). The best idear is only to add the realy neccesary fields to the materialized/indexed view.