How does a function like SUM work? If I execute
select id,sum(a) from mytable group by id
does it sort by id and then sum over each range of equal id's? I am no planner expert, but it looks like that is what is happening, where mytable is maybe a hundred million rows with a few million distinct id's.
Or does it just keep a hash of id -> current_sum, and then at each row either increments the value of id or add a new key? Isn't that far faster and less memory hungry?
SQL standards try to dictate external behavior, not internal behavior. In this particular case, a SQL implementation that conforms to (one of the many) standards is supposed to act like it does things in this order.
Build a working table from all the table constructors in the FROM clause. (There's only one in your example.)
In the GROUP BY clause, partition the working table into groups. Reduce each group to one row. Replace the working table with the grouped table.
Resolve the expressions in the SELECT clause.
Query optimizers that follow SQL standards are free to rearrange things however they like, as long as the result is the same as if it had followed those steps.
You can find more details in the answers and comments to this SO question.
So, I found this, http://helmingstay.blogspot.com/2009/06/postgresql-poetry-aggregate-median-with.html, which claims that it does indeed use the accumulator pattern. Hmmm.
