The executor takes the plan handed back by the planner/optimizer and starts processing the top node. In the case of our example (the query given in example \ref{simple_select}) the top node is a MergeJoin node.
Before any merge can be done two tuples have to be fetched (one from each subplan). So the executor recursively calls itself to process the subplans (it starts with the subplan attached to lefttree). The new top node (the top node of the left subplan) is a SeqScan node and again a tuple has to be fetched before the node itself can be processed. The executor calls itself recursively another time for the subplan attached to lefttree of the SeqScan node.
Now the new top node is a Sort node. As a sort has to be done on the whole relation, the executor starts fetching tuples from the Sort node's subplan and sorts them into a temporary relation (in memory or a file) when the Sort node is visited for the first time. (Further examinations of the Sort node will always return just one tuple from the sorted temporary relation.)
Every time the processing of the Sort node needs a new tuple the executor is recursively called for the SeqScan node attached as subplan. The relation (internally referenced by the value given in the scanrelid field) is scanned for the next tuple. If the tuple satisfies the qualification given by the tree attached to qpqual it is handed back, otherwise the next tuple is fetched until the qualification is satisfied. If the last tuple of the relation has been processed a NULL pointer is returned.
After a tuple has been handed back by the lefttree of the MergeJoin the righttree is processed in the same way. If both tuples are present the executor processes the MergeJoin node. Whenever a new tuple from one of the subplans is needed a recursive call to the executor is performed to obtain it. If a joined tuple could be created it is handed back and one complete processing of the plan tree has finished.
Now the described steps are performed once for every tuple, until a NULL pointer is returned for the processing of the MergeJoin node, indicating that we are finished.