David Rowley wrote:

> If we wanted to see the sales per product we could write
> something like this:

> SELECT p.product_code,SUM(s.quantity)
> FROM products p
> INNER JOIN bigsalestable s ON p.productid = s.productid
> GROUP BY p.product_code;

> Though this plan might not be quite as optimal as it could be as
> it performs the grouping after the join.

> Of course the query could have been written in the first place
> as:

> SELECT p.product_code,s.quantity
> FROM products AS p
> INNER JOIN (SELECT productid,SUM(quantity) AS quantity
>             FROM bigsalestable GROUP BY productid) AS s
>   ON p.productid = s.productid;

> And that would have given us a more efficient plan.

> Of course, for these actual plans to be equivalent there would
> naturally have to be a unique index on product_code in the
> products table.
> 
> I think I'm right in thinking that if a unique index exists to
> match the group by clause, and the join condition is equality
> (probably using the same operator class as the unique btree
> index?), then the grouping could be pushed up to before the join.

Off-hand, it seems equivalent to me; I don't know how much work it
would be.

Out of curiosity, does the first query's plan change if you run
this instead?:

SELECT s.product_code,SUM(s.quantity)
FROM products p
INNER JOIN bigsalestable s ON p.productid = s.productid
GROUP BY s.product_code;

-Kevin

> From: Kevin Grittner [mailto:kgrittn(at)mail(dot)com]
> > I think I'm right in thinking that if a unique index exists to match
> > the group by clause, and the join condition is equality (probably
> > using the same operator class as the unique btree index?), then the
> > grouping could be pushed up to before the join.
> 
> Off-hand, it seems equivalent to me; I don't know how much work it would
> be.
> 
> Out of curiosity, does the first query's plan change if you run this instead?:
> 
> SELECT s.product_code,SUM(s.quantity)
> FROM products p
> INNER JOIN bigsalestable s ON p.productid = s.productid GROUP BY
> s.product_code;
> 

I should have made it more clear about the lack of product_code in the bigsalestable, there's only productid, the lookup to the product table was to obtain the actual more user friendly product_code.

The actual table def's are in the attachment of the original email.

Regards

David Rowley

> -Kevin