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?
The following test should demonstrate what I think.
demo=# create table t(a int); CREATE TABLE demo=# insert into t select generate_series(1, 10000000); INSERT 0 10000000 demo=# create index t_a on t(a); CREATE INDEX demo=# analyze t; ANALYZE demo=# set enable_seqscan to off; SET demo=# set enable_indexscan to off; SET demo=# set enable_bitmapscan to off; SET demo=# set enable_indexonlyscan to off; SET demo=# explain select * from t where a = 1; QUERY PLAN --------------------------------------------------------------------------------- Index Scan using t_a on t (cost=10000000000.43..10000000008.45 rows=1 width=4) Index Cond: (a = 1) (2 rows)
If we just disable index path, we will get seqscan at last.
