From d7652b6664ac1c965ba86d374a94fdeb7691fbb1 Mon Sep 17 00:00:00 2001 From: Andrey Borodin Date: Tue, 16 Jan 2018 21:37:51 +0500 Subject: [PATCH 3/3] Update README with info on new GiST VACUUM --- src/backend/access/gist/README | 35 +++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) diff --git a/src/backend/access/gist/README b/src/backend/access/gist/README index 02228662b8..6b2b12ee96 100644 --- a/src/backend/access/gist/README +++ b/src/backend/access/gist/README @@ -413,6 +413,41 @@ emptied yet; tuples never move upwards in the tree. The final emptying loops through buffers at a given level until all buffers at that level have been emptied, and then moves down to the next level. +Bulk delete algorithm (VACUUM) +------------------------------ + +Function gistbulkdelete() is responsible for marking empty leaf pages as free +so that they can be used it allocate newly split pages. To find this pages +function can choose between two strategies: logical scan or physical scan. + +Physical scan reads the entire index from the first page to last. This scan +maintains graph structure in palloc'ed array to collect block numbers of +internal pages that need cleansing from references to empty leafs. Also, the +array contains offsets on the internal page to potentially free leaf page. This +scan method is chosen when maintenance work memory is sufficient to hold +necessary graph structure. + +The logical scan is chosen when there is not enough maintenance memory to +execute the physical scan. Logical scan traverses GiST index in BFS, looking up +into incomplete split branches. The logical scan can be slower on hard disk +drives. + +The result of both scans are the same: the stack of block numbers of internal +pages with the list of offsets potentially referencing empty leaf pages. After +the scan, for each internal pages under exclusive lock, each potentially free +leaf page is examined. gistbulkdelete() never delete last one reference from +internal page to preserve balanced tree properties. + +The physical scan can return empty leaf pages offsets unordered. Thus, before +executing PageIndexMultiDelete offsets (already locked and checked) are sorted. +This step is not necessary for the logical scan. + +Both scans hold only one lock at a time. Physical scan grabs exclusive lock +instantly, while logical scan takes shared lock and then swaps it to exclusive. +This is done because amount of work on internal page done by physical scan is +lower and amount of internal pages is relatively low compared to the amount of +leaf pages. + Authors: Teodor Sigaev -- 2.14.3 (Apple Git-98)