Re: [PATCH] Erase the distinctClause if the result is unique by definition - Mailing list pgsql-hackers

Hi Ashutosh:

   Thanks for your time. 

On Fri, Feb 7, 2020 at 11:54 PM Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> wrote:

Hi Andy,

What might help is to add more description to your email message like giving examples to explain your idea.

Anyway, I looked at the testcases you added for examples.

+create table select_distinct_a(a int, b char(20),  c char(20) not null,  d int, e int, primary key(a, b));
+set enable_mergejoin to off;
+set enable_hashjoin to off;
+-- no node for distinct.
+explain (costs off) select distinct * from select_distinct_a;
+          QUERY PLAN          
+-------------------------------
+ Seq Scan on select_distinct_a
+(1 row)

From this example, it seems that the distinct operation can be dropped because (a, b) is a primary key. Is my understanding correct?

Yes, you are correct.   Actually I added then to commit message,

but it's true that I should have copied them in this email body as

 well.  so copy it now. 

[PATCH] Erase the distinctClause if the result is unique by
 definition

However the patch as presented has some problems

1. What happens if the primary key constraint or NOT NULL constraint gets dropped between a prepare and execute? The plan will no more be valid and thus execution may produce non-distinct results.

Will this still be an issue if user use doesn't use a "read uncommitted" 

isolation level?  I suppose it should be ok for this case.  But even though

I should add an isolation level check for this.  Just added that in the patch

to continue discussing of this issue. 

 

PostgreSQL has similar concept of allowing non-grouping expression as part of targetlist when those expressions can be proved to be functionally dependent on the GROUP BY clause. See check_functional_grouping() and its caller. I think, DISTINCT elimination should work on similar lines.

2. For the same reason described in check_functional_grouping(), using unique indexes for eliminating DISTINCT should be discouraged.

 

I checked the comments of check_functional_grouping,  the reason is 

 * Currently we only check to see if the rel has a primary key that is a
 * subset of the grouping_columns.  We could also use plain unique constraints
 * if all their columns are known not null, but there's a problem: we need
 * to be able to represent the not-null-ness as part of the constraints added
 * to *constraintDeps.  FIXME whenever not-null constraints get represented
 * in pg_constraint.

Actually I am doubtful the reason for pg_constraint since we still be able 

to get the not null information from relation->rd_attr->attrs[n].attnotnull which 

is just what this patch did.   

3. If you could eliminate DISTINCT you could similarly eliminate GROUP BY as well

This is a good point.   The rules may have some different for join,  so I prefer 

to to focus on the current one so far.

 

4. The patch works only at the query level, but that functionality can be expanded generally to other places which add Unique/HashAggregate/Group nodes if the underlying relation can be proved to produce distinct rows. But that's probably more work since we will have to label paths with unique keys similar to pathkeys.

 

Do you mean adding some information into PlannerInfo,  and when we create 

a node for Unique/HashAggregate/Group,  we can just create a dummy node?