database/mariadb
1
0
Fork 0
mirror of https://github.com/MariaDB/server.git synced 2025-07-29 05:21:33 +03:00
Code Activity

Bug #25551: inconsistent behaviour in grouping NULL, depending on index type

Browse Source 
The optimizer takes away columns from GROUP BY/DISTINCT if they constitute
 all the parts of an unique index.
 However if some of the columns can contain NULLs this cannot be done 
(because an UNIQUE index can have multiple rows with NULL values).
 Fixed by not using UNIQUE indexes with nullable columns to remove
 grouping columns from GROUP BY/DISTINCT.
This commit is contained in:
gkodinov/kgeorge@macbook.gmz
2007-01-31 10:18:26 +02:00
parent b8d55cc449
commit 2ba683f2ee
3 changed files with 52 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

19
sql/sql_select.cc
View File

@ -848,10 +848,11 @@ JOIN::optimize()
}
/*
Check if we can optimize away GROUP BY/DISTINCT.
We can do that if there are no aggregate functions and the
We can do that if there are no aggregate functions, the
fields in DISTINCT clause (if present) and/or columns in GROUP BY
(if present) contain direct references to all key parts of
an unique index (in whatever order).
an unique index (in whatever order) and if the key parts of the
unique index cannot contain NULLs.
Note that the unique keys for DISTINCT and GROUP BY should not
be the same (as long as they are unique).
@ -11856,7 +11857,7 @@ test_if_subkey(ORDER *order, TABLE *table, uint ref, uint ref_key_parts,
/*
Check if GROUP BY/DISTINCT can be optimized away because the set is
Check if GROUP BY/DISTINCT can be optimized away because the set is
already known to be distinct.
SYNOPSIS
@ -11864,7 +11865,7 @@ test_if_subkey(ORDER *order, TABLE *table, uint ref, uint ref_key_parts,
table The table to operate on.
find_func function to iterate over the list and search
for a field
DESCRIPTION
Used in removing the GROUP BY/DISTINCT of the following types of
statements:
@ -11875,12 +11876,13 @@ test_if_subkey(ORDER *order, TABLE *table, uint ref, uint ref_key_parts,
then <any combination of a,b,c>,{whatever} is also distinct
This function checks if all the key parts of any of the unique keys
of the table are referenced by a list : either the select list
of the table are referenced by a list : either the select list
through find_field_in_item_list or GROUP BY list through
find_field_in_order_list.
If the above holds then we can safely remove the GROUP BY/DISTINCT,
If the above holds and the key parts cannot contain NULLs then we
can safely remove the GROUP BY/DISTINCT,
as no result set can be more distinct than an unique key.
RETURN VALUE
1 found
0 not found.
@ -11903,7 +11905,8 @@ list_contains_unique_index(TABLE *table,
key_part < key_part_end;
key_part++)
{
if (!find_func(key_part->field, data))
if (key_part->field->maybe_null() ||
!find_func(key_part->field, data))
break;
}
if (key_part == key_part_end)