Thanks, Dmitry.

> For inner SORTs, it depends on their positions inside a plan.

Does this mean the inner SORTs might require extra space ?

> Calculation is a bit tricky. What's your row format and number of rows
> (before grouping)?

There's 3 individual columns of CHAR(10), 5 aggregate columns of
NUMERIC(15, 4). Before grouping, about 10 million rows and after
grouped by the 3 individual columns, it's about 0.6 million.