One you have a set of rows, the next type of node you'll encounter when using a single table are ones that process that set in various ways. These nodes typically take in a row set and output a different row set, of either the same size or smaller (perhaps only a single value).
Sort nodes can appear when you insert
ORDER BY statements into your queries:
EXPLAIN ANALYZE SELECT customerid FROM customers ORDER BY zip; QUERY PLAN ---------- Sort (cost=2104.77..2154.77 rows=20000 width=8) (actual time=162.796..199.971 rows=20000 loops=1) Sort Key: zip Sort Method: external sort Disk: 352kB -> Seq Scan on customers (cost=0.00..676.00 rows=20000 width=8) (actual time=0.013..46.748 rows=20000 loops=1) Total runtime: 234.527 ms ...