Improving speed using clustered tables

In this section, you will learn about the power of correlation and clustered tables. What is this about? Imagine that you want to read a whole area of data. This might be a certain time range, a block, some IDs, and so on.

The runtime of these 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
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