Hitoshi Harada <umi.tan...@gmail.com> writes:
> I wonder why SortState is a ScanState. As far as I know ScanState
> means the node may need projection and/or qualification, or it scans
> some relation, but Sort actually doesn't do such things.

No, not really.  Per the comment for ScanState:

 *        ScanState extends PlanState for node types that represent
 *        scans of an underlying relation.  It can also be used for nodes
 *        that scan the output of an underlying plan node --- in that case,
 *        only ScanTupleSlot is actually useful, and it refers to the tuple
 *        retrieved from the subplan.

It might be that we don't actually need ScanTupleSlot right now in the
implementation of Sort, but I don't see a good reason to remove the
field.  We might just have to put it back later.

BTW, Sort is not the only node type like this --- I see at least
Material that's not projection-capable but has a ScanState.

                        regards, tom lane

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