why sqlite use freelist leaf page as it contains no information

Question

According to the SQLite document https://www.sqlite.org/fileformat.html, the function of Freelist pages is as follows:

A database file might contain one or more pages that are not in active use. Unused pages can come about, for example, when information is deleted from the database. Unused pages are stored on the freelist and are reused when additional pages are required.

The freelist is organized as a linked list of freelist trunk pages with each trunk page containing page numbers for zero or more freelist leaf pages.

A freelist trunk page consists of an array of 4-byte big-endian integers. The size of the array is as many integers as will fit in the usable space of a page. The minimum usable space is 480 bytes so the array will always be at least 120 entries in length. The first integer on a freelist trunk page is the page number of the next freelist trunk page in the list or zero if this is the last freelist trunk page. The second integer on a freelist trunk page is the number of leaf page pointers to follow. Call the second integer on a freelist trunk page L. If L is greater than zero then integers with array indexes between 2 and L+1 inclusive contain page numbers for freelist leaf pages.

Freelist leaf pages contain no information. SQLite avoids reading or writing freelist leaf pages in order to reduce disk I/O.

A bug in SQLite versions prior to 3.6.0 (2008-07-16) caused the database to be reported as corrupt if any of the last 6 entries in the freelist trunk page array contained non-zero values. Newer versions of SQLite do not have this problem. However, newer versions of SQLite still avoid using the last six entries in the freelist trunk page array in order that database files created by newer versions of SQLite can be read by older versions of SQLite.

The number of freelist pages is stored as a 4-byte big-endian integer in the database header at an offset of 36 from the beginning of the file. The database header also stores the page number of the first freelist trunk page as a 4-byte big-endian integer at an offset of 32 from the beginning of the file.

I'm curious about this sentence:

Freelist leaf pages contain no information. SQLite avoids reading or writing freelist leaf pages in order to reduce disk I/O.

Since it contains no information, why does SQLite have to use it? Is there any information that I missed?

Answer 1

The freelist leaf pages are the empty, currently unused ones that will be taken off the list and merged into a b-tree in the database when needed. All that needs to be tracked is their existence and location in the file, which the freelist trunk page(s) do. The actual contents of those pages is irrelevant until they're needed, and then it will just write the new page data to the right spot.