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https://issues.apache.org/jira/browse/PHOENIX-4690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16440461#comment-16440461
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chenglei edited comment on PHOENIX-4690 at 4/17/18 6:26 AM:
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[~jamestaylor] , thank you for the review.
bq.Orthogonal to this, but it'd be good if we had a way to know if it's better 
to have the GROUP BY be order preserving followed by a client-side sort, or for 
the GROUP BY to be non order preserving with a merge sort on the client. I 
think we need histograms to make that determination.

In my opinion, I think GROUP BY be order preserving is always better than GROUP 
BY to be non order preserving,because if  GROUP BY is non order preserving, in 
additional to a merge sort on the client, we need a client-side sort on the 
server-side hash aggregated results before the merge sort. 


was (Author: comnetwork):
[~jamestaylor] , thank you for the review.
bq.Orthogonal to this, but it'd be good if we had a way to know if it's better 
to have the GROUP BY be order preserving followed by a client-side sort, or for 
the GROUP BY to be non order preserving with a merge sort on the client. I 
think we need histograms to make that determination.

In my opinion, I think GROUP BY be order preserving is always better than GROUP 
BY to be non order preserving,because if  GROUP BY is non order preserving, in 
additional to we need a client-side sort on the server-side hash aggregated 
results  

> GroupBy expressions should follow the order of PK Columns if GroupBy is 
> orderPreserving
> ---------------------------------------------------------------------------------------
>
>                 Key: PHOENIX-4690
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-4690
>             Project: Phoenix
>          Issue Type: Bug
>    Affects Versions: 4.14.0, 4.13.2
>            Reporter: chenglei
>            Assignee: chenglei
>            Priority: Critical
>             Fix For: 4.14.0, 5.0.0
>
>         Attachments: PHOENIX-4690_v1.patch
>
>
> Given a table :
> {code}
>  create table test ( 
>        pk1 integer not null , 
>        pk2 integer not null, 
>        v integer, 
>        CONSTRAINT TEST_PK PRIMARY KEY (pk1,pk2))
> {code}
> and some data:
> {code}
> +------+------+-----+
> | PK1  | PK2  |  V  |
> +------+------+-----+
> | 1    | 8    | 10  |
> | 1    | 9    | 11  |
> | 2    | 3    | 13  |
> | 2    | 7    | 15  |
> | 3    | 2    | 17  |
> +------+------+-----+
> {code}
> for following sql :
> {code}
>     select pk2,pk1,count(v) from test group by pk2,pk1 order by pk2,pk1
> {code}
> the expected result is :
> {code}
> +------+------+-----------+
> | PK2  | PK1  | COUNT(V)  |
> +------+------+-----------+
> | 2    | 3    | 1         |
> | 3    | 2    | 1         |
> | 7    | 2    | 1         |
> | 8    | 1    | 1         |
> | 9    | 1    | 1         |
> +------+------+-----------+
> {code}
> but the actual result is :
> {code}
> +------+------+-----------+
> | PK2  | PK1  | COUNT(V)  |
> +------+------+-----------+
> | 8    | 1    | 1         |
> | 9    | 1    | 1         |
> | 3    | 2    | 1         |
> | 7    | 2    | 1         |
> | 2    | 3    | 1         |
> +------+------+-----------+
> {code}
> The problem is caused by the {{GroupBy.compile}}, obviously, in line 154, for 
> {{group by pk2,pk1}},
> {{OrderPreservingTracker.isOrderPreserving}} is true by reorder the 
> {{pk2,pk1}} to {{pk1,pk2}} following the order of PK columns {{pk1,pk2}},  
> but in line 158, for the new GroupBy, the GroupBy.expressions is still 
> {{pk2,pk1}},not the actual {{pk1,pk2}}:
> {code}
> 141       public GroupBy compile(StatementContext context, TupleProjector 
> tupleProjector) throws SQLException {
> 142              boolean isOrderPreserving = this.isOrderPreserving;
> 143              int orderPreservingColumnCount = 0;
> 144              if (isOrderPreserving) {
> 145                    OrderPreservingTracker tracker = new 
> OrderPreservingTracker(context, GroupBy.EMPTY_GROUP_BY, Ordering.UNORDERED, 
> expressions.size(), tupleProjector);
> 146              for (int i = 0; i < expressions.size(); i++) {
> 147                    Expression expression = expressions.get(i);
> 148                    tracker.track(expression);
> 149                }
> 150                
> 151                // This is true if the GROUP BY is composed of only PK 
> columns. We further check here that
> 152                // there are no "gaps" in the PK columns positions used 
> (i.e. we start with the first PK
> 153                // column and use each subsequent one in PK order).
> 154                isOrderPreserving = tracker.isOrderPreserving();
> 155                orderPreservingColumnCount = 
> tracker.getOrderPreservingColumnCount();
> 156            }
> 157            if (isOrderPreserving || isUngroupedAggregate) {
> 158                return new 
> GroupBy.GroupByBuilder(this).setIsOrderPreserving(isOrderPreserving)
> .setOrderPreservingColumnCount(orderPreservingColumnCount).build();
> 159     }
> {code}
> Then when we compile {{order by pk2,pk1}} in {{OrderByCompiler.compile}}, 
> because {{the GroupBy.isOrderPreserving}} is true, and {{order by pk2,pk1}} 
> is consistent with the GroupBy.expressions {{group by pk2,pk1}} created in 
> above  {{GroupBy.compile}} method, so the result of  
> {{OrderByCompiler.compile}} is {{OrderBy.FWD_ROW_KEY_ORDER_BY}}.
> But in fact,because the actual GroupBy.expressions is {{group by pk1,pk2}},so 
> we need to execute  {{order by pk2,pk1}} after the {{group by pk1,pk2}}.



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