In this section, you will learn about the power of correlation and the power of clustered tables. What is the whole idea? Consider you want to read a whole area of data. This might be a certain time range, some block, IDs, or so.
The runtime of such queries will vary depending on the amount of data and the physical arrangement of data on the disk. So, even if you are running queries that return the same number of rows, two systems might not provide the answer within the same time span, as the physical disk layout might make a difference.
Here is an example:
test=# EXPLAIN (analyze true, buffers true, timing true)
SELECT *
FROM t_test WHERE id < 10000;
QUERY PLAN
----------------------------------------------------------
Index Scan using idx_id on t_test
(cost=0.43..370.87 rows=10768 width=9)
(actual...