On Tue, May 26, 2020 at 1:52 PM Andy Fan <zhihui.fan1213@gmail.com> wrote:
>
>
> Consider the below example:
>
> create table j1(i int, im5 int, im100 int, im1000 int);
> insert into j1 select i, i%5, i%100, i%1000 from generate_series(1, 10000000)i;
> create index j1_i_im5 on j1(i, im5);
> create index j1_i_im100 on j1(i, im100);
> analyze j1;
> explain select * from j1 where i = 100 and im5 = 5;
>
> We may get the plan like this:
>
> demo=# explain select * from j1 where i = 100 and im5 = 1;
> QUERY PLAN
> ----------------------------------------------------------------------
> Index Scan using j1_i_im100 on j1 (cost=0.43..8.46 rows=1 width=16)
> Index Cond: (i = 100)
> Filter: (im5 = 1)
> (3 rows)
>
> At this case, optimizer can estimate there are only 1 row to return, so both
> indexes have same cost, which one will be choose is un-controlable. This is
> fine for above query based on the estimation is accurate. However estimation
> can't be always accurate in real life. Some inaccurate estimation can cause an
> wrong index choose. As an experience, j1_i_im5 index should always be choose
> for above query.
I think we need a better example where choosing an index makes a difference.
An index can be chosen just because it's path was created before some
other more appropriate index but the cost difference was within fuzzy
limit. Purely based on the order in which index paths are created.
>
> This one line change is the best method I can think.
>
> - cpu_per_tuple = cpu_tuple_cost + qpqual_cost.per_tuple;
> + cpu_per_tuple = cpu_tuple_cost + (qpqual_cost.per_tuple * 1.001);
>
> We make the qual cost on index filter is slightly higher than qual cost in Index
> Cond. This will also good for QUAL (i=x AND m=y AND n=z). Index are (i, m,
> other_col1) and (i, other_col1, other_col2). But this change also
> changed the relation between the qual cost on index scan and qual cost on seq
> scan. However I think that impact is so tiny that I think we can ignore that (we
> can choose a better factor between 1 and 1.001).
>
> Even the root cause of this issue comes from an inaccurate estimation. but I
> don't think that is an issue easy/possible to fix, however I'm open for
> suggestion on that as well.
>
> Any suggestions?
>
> --
> Best Regards
> Andy Fan
--
Best Wishes,
Ashutosh Bapat