On Tue, Apr 14, 2020 at 5:29 PM Andy Fan <zhihui.fan1213@gmail.com> wrote: > On Tue, Apr 14, 2020 at 3:40 PM Amit Langote <amitlangote09@gmail.com> wrote: >> On Tue, Apr 14, 2020 at 4:13 PM Richard Guo <guofenglinux@gmail.com> wrote: >> > On Tue, Apr 14, 2020 at 2:44 PM Amit Langote <amitlangote09@gmail.com> wrote: >> >> Maybe I am missing something obvious, but is it intentional that >> >> enable_indexscan is checked by cost_index(), that is, *after* creating >> >> an index path? I was expecting that if enable_indexscan is off, then >> >> no index paths would be generated to begin with, because I thought >> >> they are optional. >> > >> > I think the cost estimate of index paths is the same as other paths on >> > that setting enable_xxx to off only adds a penalty factor (disable_cost) >> > to the path's cost. The path would be still generated and compete with >> > other paths in add_path(). >> >> Yeah, but I am asking why build the path to begin with, as there will >> always be seq scan path for base rels. > > I guess that is because user may disable seqscan as well. If so, we > still need formula to decide with one to use, which requires index path > has to be calculated. but since disabling the two at the same time is rare, > we can ignore the index path build if user allow seqscan
I am saying that instead of building index path with disabled cost, just don't build it at all. A base rel will always have a sequetial path, even though with disabled cost if enable_seqscan = off.
Let's say user set enable_seqscan=off and set enable_indexscan=off;
will you expect user to get seqscan at last? If so, why is seqscan
(rather than index scan) since both are disabled by user equally?
