Dead tuples, the free space map and why VACUUM almost never gives disk space back.
This is one of the sample posts that come with the Notebook theme. Write your own in Posts > Add New: give it a title, an excerpt (the summary under the title), a series (category, or Notes), tags and the text.
Headings in the text make up the post's "On this page" outline, and every Code block gets a copy button. Give a Code block the class lang-sql (Advanced > Additional CSS class) to label it SQL.