> If 1000 rows deleted by a commited transaction, is the space available
> immediately or only after GC runs?

After GC run, but not necessary. These rows aren't garbage as long as
any transaction needs them.

> Does the GC starts automatically (configurable?) or/and is it possible to
> start GC manually (without running sweep)?

GC strategy is configurable, but it always is started automatically
as soon as any reading transaction (including sweeping one) encounter
garbage.

> Is GC/sweeping required for fast select queries?

No. It rather depends... but generally - no.
For fast select queries straight hands of DB and application
developers are required.

SY, SD.