This web content explains the structure and functionality of a 3-level B+ tree. The tree consists of branch pages and leaf pages, with data and keys stored in the leaf pages. Leaf chaining, which allows for easy sequential access between leaf pages, is not supported as it requires rewriting the entire tree with each update. The pages are stored sequentially in the database file, with increasing page numbers indicating increasing file offsets. The meta page contains important information such as a pointer to the root page, a hash, and statistics counters. When the file is opened, it is scanned backwards to locate a valid meta page and thus find the root of the tree.
When updating a value in a leaf page, instead of changing the page in-place, a new page is appended to the file. This change in location requires updating the parent pages to point to the new locations. However, existing pages are never modified, and new pages are only appended to the file. A new root page is created as a copy of the previous root page, with the only change being the update of the pointer to the branch page. This allows cursors that still have pointers to the original root page to traverse the tree without being affected by the change, ensuring a consistent snapshot of the
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