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efada2b8e9
In short, we don't allow a page to be deleted if it's the rightmost child of its parent, but that situation can change after we check for it. Problem ------- We check that the page to be deleted is not the rightmost child of its parent, and then lock its left sibling, the page itself, its right sibling, and the parent, in that order. However, if the parent page is split after the check but before acquiring the locks, the target page might become the rightmost child, if the split happens at the right place. That leads to an error in vacuum (I reproduced this by setting a breakpoint in debugger): ERROR: failed to delete rightmost child 41 of block 3 in index "foo_pkey" We currently re-check that the page is still the rightmost child, and throw the above error if it's not. We could easily just give up rather than throw an error, but that approach doesn't scale to half-dead pages. To recap, although we don't normally allow deleting the rightmost child, if the page is the *only* child of its parent, we delete the child page and mark the parent page as half-dead in one atomic operation. But before we do that, we check that the parent can later be deleted, by checking that it in turn is not the rightmost child of the grandparent (potentially recursing all the way up to the root). But the same situation can arise there - the grandparent can be split while we're not holding the locks. We end up with a half-dead page that we cannot delete. To make things worse, the keyspace of the deleted page has already been transferred to its right sibling. As the README points out, the keyspace at the grandparent level is "out-of-whack" until the half-dead page is deleted, and if enough tuples with keys in the transferred keyspace are inserted, the page might get split and a downlink might be inserted into the grandparent that is out-of-order. That might not cause any serious problem if it's transient (as the README ponders), but is surely bad if it stays that way. Solution -------- This patch changes the page deletion algorithm to avoid that problem. After checking that the topmost page in the chain of to-be-deleted pages is not the rightmost child of its parent, and then deleting the pages from bottom up, unlink the pages from top to bottom. This way, the intermediate stages are similar to the intermediate stages in page splitting, and there is no transient stage where the keyspace is "out-of-whack". The topmost page in the to-be-deleted chain doesn't have a downlink pointing to it, like a page split before the downlink has been inserted. This also allows us to get rid of the cleanup step after WAL recovery, if we crash during page deletion. The deletion will be continued at next VACUUM, but the tree is consistent for searches and insertions at every step. This bug is old, all supported versions are affected, but this patch is too big to back-patch (and changes the WAL record formats of related records). We have not heard any reports of the bug from users, so clearly it's not easy to bump into. Maybe backpatch later, after this has had some field testing. Reviewed by Kevin Grittner and Peter Geoghegan. |
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PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.