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Re: 5e16a49f5d9: MDEV-26013 distinct not work properly in some cases for spider tables


Hi, Nayuta!

On Jul 21, Nayuta Yanagisawa wrote:
> Hi Sergei,
> Thank you for your review!
> > If DISTINCT was coverted to a GROUP BY, why would the engine need to
> > know whether there was DISTINCT or not originally? There is no
> > DISTINCT on the execution plan now, that should be sufficient,
> > shouldn't it?
> > Why does the query fail with GROUP BY?
> For the select query in the test case (SELECT distinct b FROM tbl_a WHERE b=999),
> the optimizer seems to convert DISTINCT to GROUP by and then optimize away
> GROUP BY. The, we get select_distinct = 0, no_order = 1, group_optimized_away = 1.
> Please see sql/sql_select.cc:2721-2781.
> In such a case, group_list is NULL and thus the Spider SE misunderstand that
> the query has neither DISTINCT and GROUP BY without my fix.

Oh, I see. group_list is NULL, I missed that.

  select distinct b from tbl_a where b=999;

see, it asks for distinct values of `b` for b=999. There can be only one
row in this query. Or none. So the optimizer changes DISTINCT to an
implicit GROUP BY, similar to `select count(*) from tbl_a` -
such an implicit GROUP BY can return at most one row.

I agree, it is wrong that implicit GROUP BY has no place in the Query
structure. Does Spider work correctly for, say, `SELECT COUNT(*)` or
`SELECT SUM(a)` - that is, for other cases of such implicit GROUP BY?

If it does, it likely deducts the need for GROUP BY when seeing COUNT()
or SUM() in the select list. Which doesn't work for converted DISTINCT.

In that case, I think your fix is good. One question: why `no_order &&` ?

VP of MariaDB Server Engineering
and security@xxxxxxxxxxx

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