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
> 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
Out of curiosity, does the first query's plan change if you run
FROM products p
INNER JOIN bigsalestable s ON p.productid = s.productid
GROUP BY s.product_code;