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Bug#12328597 - MULTIPLE COUNT(DISTINCT) IN SAME SELECT FALSE

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WITH COMPOSITE KEY COLUMNS

Problem:-
While running a SELECT query with several AGGR(DISTINCT) function 
and these are referring to different field of same composite key, 
Returned incorrect value.

Analysis:-

In a table, where we have composite key like (a,b,c)
and when we give a query like

select COUNT(DISTINCT b), SUM(DISTINCT a) from ....

here, we first make a list of items in Aggr(distinct) function
(which is a, b), where order of item doesn't matter. 
and then we see, whether we have a composite key where the prefix 
of index columns matches the items of the aggregation function.
(in this case we have a,b,c).

if yes, so we can use loose index scan and we need not perform 
duplicate removal to distinct in our aggregate function.

In our table, we traverse column marked with <-- and get the result as
(a,b,c)      count(distinct b)           sum(distinct a)
             treated as count b          treated as sum(a)
(1,1,2)<--              1                      1		
(1,2,2)<--              1++=2                  1+1=2
(1,2,3)		
(2,1,2)<--              2++=3                  1+1+2=4
(2,2,2)<--              3++=4                  1+1+2+2=6
(2,2,3)

result will be 4,6, but it should be (2,3)

As in this case, our assumption is incorrect. If we have
query like 
select count(distinct a,b), sum(distinct a,b)from ..
then we can use loose index scan

Solution:-
In our query, when we have more then one aggr(distinct) function 
then they should refer to same  fields like

select count(distinct a,b), sum(distinct a,b) from .. 

-->we can use loose scan index as both aggr(distinct) refer to same fields a,b.

If they are referring to different field like

select count(distinct a), sum(distinct b) from .. 

-->will not use loose scan index as both aggr(distinct) refer to different fields.
This commit is contained in:
Neeraj Bisht
2013-05-13 17:15:25 +05:30
parent 05111d32ca
commit 35a3f9d76c
5 changed files with 40 additions and 8 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
mysql-test/r/group_min_max.result
View File

@ -2858,9 +2858,10 @@ COUNT(DISTINCT a, b + 0)
16
EXPLAIN SELECT COUNT(DISTINCT a) FROM t1 HAVING COUNT(DISTINCT b) < 10;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range NULL a 10 NULL 9 Using index for group-by
1 SIMPLE t1 index NULL a 10 NULL 16 Using index
SELECT COUNT(DISTINCT a) FROM t1 HAVING COUNT(DISTINCT b) < 10;
COUNT(DISTINCT a)
2
EXPLAIN SELECT COUNT(DISTINCT a) FROM t1 HAVING COUNT(DISTINCT c) < 10;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 16
@ -2994,7 +2995,7 @@ WHERE b = 13 AND c = 42 GROUP BY a;
a COUNT(DISTINCT a) SUM(DISTINCT a)
EXPLAIN SELECT COUNT(DISTINCT a, b), SUM(DISTINCT a) FROM t2 WHERE b = 42;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t2 range NULL a 10 NULL 9 Using where; Using index for group-by
1 SIMPLE t2 index NULL a 15 NULL 16 Using where; Using index
SELECT COUNT(DISTINCT a, b), SUM(DISTINCT a) FROM t2 WHERE b = 42;
COUNT(DISTINCT a, b) SUM(DISTINCT a)
0 NULL

2
mysql-test/t/group_min_max.test
View File

@ -1207,6 +1207,8 @@ EXPLAIN SELECT a, COUNT(DISTINCT a), SUM(DISTINCT a) FROM t2
SELECT a, COUNT(DISTINCT a), SUM(DISTINCT a) FROM t2
WHERE b = 13 AND c = 42 GROUP BY a;
# This query could have been resolved using loose index scan since the second
# part of count(..) is defined by a constant predicate
EXPLAIN SELECT COUNT(DISTINCT a, b), SUM(DISTINCT a) FROM t2 WHERE b = 42;
SELECT COUNT(DISTINCT a, b), SUM(DISTINCT a) FROM t2 WHERE b = 42;