Em sex., 8 de mai. de 2020 às 19:02, Tomas Vondra < tomas.von...@2ndquadrant.com> escreveu:
> On Thu, Apr 23, 2020 at 04:12:34PM -0400, James Coleman wrote: > >On Thu, Apr 23, 2020 at 8:38 AM Ranier Vilela <ranier...@gmail.com> > wrote: > >> > >> Hi, > >> > >> Per Coverity. > >> > >> If has 0 full groups, "we don't need to do anything" and need goes to > next. > >> Otherwise a integer division by zero, can raise. > >> > >> comments extracted trom explain.c: > >> /* > >> * Since we never have any prefix groups unless we've first sorted > >> * a full groups and transitioned modes (copying the tuples into a > >> * prefix group), we don't need to do anything if there were 0 full > >> * groups. > >> */ > > > >This does look like a fairly obvious thinko on my part, and the patch > >looks correct to me. > > > >Tomas: agreed? > > > > So how do we actually get the division by zero? It seems to me the fix > prevents a division by zero with 0 full groups and >0 prefix groups, > but can that actually happen? > > But can that actually happen? Doesn't the comment quoted in the report > actually suggest otherwise? If this > > (fullsortGroupInfo->groupCount == 0 && > prefixsortGroupInfo->groupCount == 0) > > First this line, contradicts the comments. According to the comments, if ( fullsortGroupInfo->groupCount == 0) is true, there is no need to do anything else, next. So anyway, we don't need to test anything anymore. Now, to happen the division by zero, (prefixsortGroupInfo->groupCount == 0, needs to be true too, Maybe this is not happening, but if it happens, it divides by zero, just below, so if an unnecessary test and adds a risk, why not, remove it? > evaluates to false, and > > (fullsortGroupInfo->groupCount == 0) > > this evaluates to true, then clearly there would have to be 0 full > groups and >0 prefix groups. But the comment says that can't happen, > unless I misunderstand what it's saying. > Comments says: "we don't need to do anything if there were 0 full groups." regards, Ranier Vilela
