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It's standard for quicksort implementations, after having partitioned the input into two subgroups, to recurse to process the smaller partition and then handle the larger partition by iterating. This method guarantees that no more than log2(N) levels of recursion can be needed. However, Bentley and McIlroy argued that checking to see which partition is smaller isn't worth the cycles, and so their code doesn't do that but just always recurses on the left partition. In most cases that's fine; but with worst-case input we might need O(N) levels of recursion, and that means that qsort could be driven to stack overflow. Such an overflow seems to be the only explanation for today's report from Yiqing Jin of a SIGSEGV in med3_tuple while creating an index of a couple billion entries with a very large maintenance_work_mem setting. Therefore, let's spend the few additional cycles and lines of code needed to choose the smaller partition for recursion. Also, fix up the qsort code so that it properly uses size_t not int for some intermediate values representing numbers of items. This would only be a live risk when sorting more than INT_MAX bytes (in qsort/qsort_arg) or tuples (in qsort_tuple), which I believe would never happen with any caller in the current core code --- but perhaps it could happen with call sites in third-party modules? In any case, this is trouble waiting to happen, and the corrected code is probably if anything shorter and faster than before, since it removes sign-extension steps that had to happen when converting between int and size_t. In passing, move a couple of CHECK_FOR_INTERRUPTS() calls so that it's not necessary to preserve the value of "r" across them, and prettify the output of gen_qsort_tuple.pl a little. Back-patch to all supported branches. The odds of hitting this issue are probably higher in 9.4 and up than before, due to the new ability to allocate sort workspaces exceeding 1GB, but there's no good reason to believe that it's impossible to crash older branches this way. |
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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/.