ruan_h wrote:

>
>> For inner SORTs, it depends on their positions inside a plan.
>
> Does this mean the inner SORTs might require extra space ?

Generally speaking -- yes, it could. In practice, it depends on the plan
details, as some inner SORTs could be released before the outer row set
is constructed entirely.

> 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.

What's charset and collation of the CHAR(10) columns? Do they belong to
the same table or different ones?


Dmitry