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Bug#17222452 - SELECT COUNT(DISTINCT A,B) INCORRECTLY COUNTS ROWS

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CONTAINING NULL

Problem:-
In MySQL, We can obtain the number of distinct expression
combinations that do not contain NULL by giving a list of 
expressions in COUNT(DISTINCT).
However rows with NULL values are
incorrectly included in the count when loose index scan is 
used.

Analysis:-
In case of loose index scan, we check whether the field is null or 
not and increase the count in Item_sum_count::add().
But there we are checking for the first field in COUNT(DISTINCT), 
not for every field. This is causing an incorrect result.

Solution:-
Check all field in Item_sum_count::add(), whether there values 
are null or not. Then only increment the count.
******
Bug#17222452 - SELECT COUNT(DISTINCT A,B) INCORRECTLY COUNTS ROWS 
	       CONTAINING NULL

Problem:-
In MySQL, We can obtain the number of distinct expression
combinations that do not contain NULL by giving a list of 
expressions in COUNT(DISTINCT).
However rows with NULL values are
incorrectly included in the count when loose index scan is 
used.

Analysis:-
In case of loose index scan, we check whether the field is null or 
not and increase the count in Item_sum_count::add().
But there we are checking for the first field in COUNT(DISTINCT), 
not for every field. This is causing an incorrect result.

Solution:-
Check all field in Item_sum_count::add(), whether there values 
are null or not. Then only increment the count.
This commit is contained in:
Neeraj Bisht 2013-09-04 10:45:55 +05:30
parent 5b333350a8
commit 6a23a44495
1 changed files with 6 additions and 2 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

8
sql/item_sum.cc
View file

@ -1496,8 +1496,12 @@ void Item_sum_count::clear()
bool Item_sum_count::add()
{
if (!args[0]->maybe_null || !args[0]->is_null())
count++;
for (uint i=0; i<arg_count; i++)
{
if (args[i]->maybe_null && args[i]->is_null())
return 0;
}
count++;
return 0;
}