Re: Why are distinct and group by choosing different plans?

"Ron Mayer" <rm_pg(at)cheapcomplexdevices(dot)com> writes:

> I notice that I get different plans when I run the
> following two queries that I thought would be
> identical.
>
> select distinct test_col from mytable;
> select test_col from mytable group by test_col;
>
> Any reason why it favors one in one case but not the other?

I think "distinct" just doesn't know about hash aggregates yet. That's partly
an oversight and partly of a "feature" in that it gives a convenient way to
write a query which avoids them. I think it's also partly that "distinct" is
trickier to fix because it's the same codepath as "distinct on" which is
decidedly more complex than a simple "distinct".

> d=# explain analyze select distinct test_col from mytable;
> QUERY PLAN
> ------------------------------------------------------------------------------------------------------------------------------------------------------------
> Unique (cost=0.00..14927.69 rows=27731 width=4) (actual time=0.144..915.214 rows=208701 loops=1)
> -> Index Scan using "mytable(test_col)" on mytable (cost=0.00..14160.38 rows=306925 width=4) (actual time=0.140..575.580 rows=306925 loops=1)
> Total runtime: 1013.657 ms
> (3 rows)

I assume you have random_page_cost dialled way down? The costs seem too low
for the default random_page_cost. This query would usually generate a sort
rather than an index scan.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com