From 49b86fb1c97878ea2e3a8118df072c95f60077ac Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas <heikki.linnakangas@iki.fi> Date: Mon, 24 Nov 2014 13:41:47 +0200 Subject: [PATCH] Add a few paragraphs to B-tree README explaining L&Y algorithm. This gives an overview of what Lehman & Yao's paper is all about, so that you can understand the rest of the README without having to read the paper. Per discussion with Peter Geoghegan and others. --- src/backend/access/nbtree/README | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README index 4820f76e3bb..213542c27f3 100644 --- a/src/backend/access/nbtree/README +++ b/src/backend/access/nbtree/README @@ -11,8 +11,25 @@ use a simplified version of the deletion logic described in Lanin and Shasha (V. Lanin and D. Shasha, A Symmetric Concurrent B-Tree Algorithm, Proceedings of 1986 Fall Joint Computer Conference, pp 380-389). -The Lehman and Yao Algorithm and Insertions -------------------------------------------- +The basic Lehman & Yao Algorithm +-------------------------------- + +Compared to a classic B-tree, L&Y adds a right-link pointer to each page, +to the page's right sibling. It also adds a "high key" to each page, which +is an upper bound on the keys that are allowed on that page. These two +additions make it possible detect a concurrent page split, which allows the +tree to be searched without holding any read locks (except to keep a single +page from being modified while reading it). + +When a search follows a downlink to a child page, it compares the page's +high key with the search key. If the search key is greater than the high +key, the page must've been split concurrently, and you must follow the +right-link to find the new page containing the key range you're looking +for. This might need to be repeated, if the page has been split more than +once. + +Differences to the Lehman & Yao algorithm +----------------------------------------- We have made the following changes in order to incorporate the L&Y algorithm into Postgres: -- GitLab