26 Sep
2016
26 Sep
'16
4:18 p.m.
On 22.09.2016 17:10, Sergei Golubchik wrote:
> Hi, Oleksandr!
>
> On Jun 28, Oleksandr Byelkin wrote:
>> revision-id: 13b5098fcaa888173472d255e29aff22bcc5baae (mariadb-10.1.13-18-g13b5098)
>> parent(s): 732adec0a4c75d99389230feeb0deca0ad668de7
>> committer: Oleksandr Byelkin
>> timestamp: 2016-06-28 10:59:59 +0200
>> message:
>>
>> MDEV-9531: GROUP_CONCAT with ORDER BY inside takes a lot of memory while it's executed
>>
>> Limitation added to Red-Black tree.
>>
>> ---
>> include/my_tree.h | 14 +++-
>> mysql-test/r/group_concat_big.result | 6 ++
>> mysql-test/t/group_concat_big.result | 6 ++
>> mysql-test/t/group_concat_big.test | 6 ++
>> mysys/tree.c | 156 ++++++++++++++++++++++++-----------
>> sql/item_sum.cc | 45 ++++++++--
>> 6 files changed, 177 insertions(+), 56 deletions(-)
>>
>> diff --git a/include/my_tree.h b/include/my_tree.h
>> index f8be55f..f1916b9 100644
>> --- a/include/my_tree.h
>> +++ b/include/my_tree.h
>> @@ -57,11 +57,14 @@ typedef struct st_tree_element {
>> } TREE_ELEMENT;
>>
>> #define ELEMENT_CHILD(element, offs) (*(TREE_ELEMENT**)((char*)element + offs))
>> +#define R_ELEMENT_CHILD(element, offs) ((TREE_ELEMENT**)((char*)element + offs))
> Please,
>
> #define ELEMENT_CHILD_PTR(element, offs) ((TREE_ELEMENT**)((char*)element + offs))
> #define ELEMENT_CHILD(element, offs) (*ELEMENT_CHILD_PTR(element, offs))
OK
>
>> typedef struct st_tree {
>> TREE_ELEMENT *root,null_element;
>> TREE_ELEMENT **parents[MAX_TREE_HEIGHT];
>> + TREE_ELEMENT *free_element;
>> uint offset_to_key,elements_in_tree,size_of_element;
>> + uint elements_limit, del_direction;
>> size_t memory_limit, allocated;
>> qsort_cmp2 compare;
>> void *custom_arg;
>> diff --git a/mysql-test/r/group_concat_big.result b/mysql-test/r/group_concat_big.result
>> new file mode 100644
>> index 0000000..4de0ebb
>> --- /dev/null
>> +++ b/mysql-test/r/group_concat_big.result
>> @@ -0,0 +1,6 @@
>> +SELECT GROUP_CONCAT( seq, seq, seq, seq, seq, seq, seq, seq ORDER BY
>> +2,1,3,4,6,5,8,7 ) AS cfield1 FROM seq_1_to_50000000;
>> +cfield1
>> +11111111,22222222,33333333,44444444,55555555,66666666,77777777,88888888,99999999,1010101010101010,1111111111111111,1212121212121212,1313131313131313,1414141414141414,1515151515151515,1616161616161616,1717171717171717,1818181818181818,1919191919191919,2020202020202020,2121212121212121,2222222222222222,2323232323232323,2424242424242424,2525252525252525,2626262626262626,2727272727272727,2828282828282828,2929292929292929,3030303030303030,3131313131313131,3232323232323232,3333333333333333,3434343434343434,3535353535353535,3636363636363636,3737373737373737,3838383838383838,3939393939393939,4040404040404040,4141414141414141,4242424242424242,4343434343434343,4444444444444444,4545454545454545,4646464646464646,4747474747474747,4848484848484848,4949494949494949,5050505050505050,5151515151515151,5252525252525252,5353535353535353,5454545454545454,5555555555555555,5656565656565656,5757575757575757,5858585858585858,5959595959595959,6060606060606060,6161616161616161,6262626262626262,63636
> 36
>> 363636363,6464646464646464,65656565
>> +Warnings:
>> +Warning 1260 Row 65 was cut by GROUP_CONCAT()
> 1. Same result without your patch?
Yes, I just checked unpatched server with this test suite , results are
the same (time is very different)
> 2. I suppose all rows after 65 skipped completely, aren't they?
yes.
>
>> diff --git a/mysql-test/t/group_concat_big.result b/mysql-test/t/group_concat_big.result
>> new file mode 100644
>> index 0000000..4de0ebb
>> --- /dev/null
>> +++ b/mysql-test/t/group_concat_big.result
>> @@ -0,0 +1,6 @@
>> +SELECT GROUP_CONCAT( seq, seq, seq, seq, seq, seq, seq, seq ORDER BY
>> +2,1,3,4,6,5,8,7 ) AS cfield1 FROM seq_1_to_50000000;
>> +cfield1
>> +11111111,22222222,33333333,44444444,55555555,66666666,77777777,88888888,99999999,1010101010101010,1111111111111111,1212121212121212,1313131313131313,1414141414141414,1515151515151515,1616161616161616,1717171717171717,1818181818181818,1919191919191919,2020202020202020,2121212121212121,2222222222222222,2323232323232323,2424242424242424,2525252525252525,2626262626262626,2727272727272727,2828282828282828,2929292929292929,3030303030303030,3131313131313131,3232323232323232,3333333333333333,3434343434343434,3535353535353535,3636363636363636,3737373737373737,3838383838383838,3939393939393939,4040404040404040,4141414141414141,4242424242424242,4343434343434343,4444444444444444,4545454545454545,4646464646464646,4747474747474747,4848484848484848,4949494949494949,5050505050505050,5151515151515151,5252525252525252,5353535353535353,5454545454545454,5555555555555555,5656565656565656,5757575757575757,5858585858585858,5959595959595959,6060606060606060,6161616161616161,6262626262626262,63636
> 36
>> 363636363,6464646464646464,65656565
>> +Warnings:
>> +Warning 1260 Row 65 was cut by GROUP_CONCAT()
> Interesting. How did you manage to include the same file twice in a
> commit diff?
It is with different path, I'll remove it.
>
>> diff --git a/mysys/tree.c b/mysys/tree.c
>> index a9fc542..6c094d9 100644
>> --- a/mysys/tree.c
>> +++ b/mysys/tree.c
>> @@ -157,6 +172,10 @@ static void free_tree(TREE *tree, myf free_flags)
>> free_root(&tree->mem_root, free_flags);
>> }
>> }
>> + if (tree->free_element && tree->with_delete && tree->free)
>> + (*tree->free)(tree->free_element, free_free,
>> + tree->custom_arg);
> "&& tree->with_delete" is wrong here
fixed
>> + tree->free_element= 0;
>> tree->root= &tree->null_element;
>> tree->elements_in_tree=0;
>> tree->allocated=0;
>> @@ -190,6 +209,42 @@ static void delete_tree_element(TREE *tree, TREE_ELEMENT *element)
>> }
>>
> function comment would be nice here
fixed.
>
>> +void tree_exclude(TREE *tree, TREE_ELEMENT ***parent)
>> +{
>> + int remove_colour;
>> + TREE_ELEMENT ***org_parent, *nod;
>> + TREE_ELEMENT *element= **parent;
>> @@ -202,7 +257,32 @@ TREE_ELEMENT *tree_insert(TREE *tree, void *key, uint key_size,
>> void* custom_arg)
>> {
>> int cmp;
>> - TREE_ELEMENT *element,***parent;
>> + TREE_ELEMENT *element, ***parent;
>> +
>> + if (tree->elements_limit && tree->elements_in_tree &&
> no need for "&& tree->elements_in_tree"
Agree.
>
>> + tree->elements_in_tree >= tree->elements_limit)
>> + {
>> + /*
>> + The limit reached so we should remove one.
>> + It is done on the very beginning because:
>> + 1) "parents" will be used
>> + 2) removing make incorrect search pass
> what does that mean?
It means that re-balancing the tree after deleting can make prepared
search pass incorrect (there is search path stored in a array).
>
>> + If we will not insert now, we leave freed element for future use
>> + */
>> + DBUG_ASSERT(key_size == 0);
> why?
I am not sure that it will work correctly when we store key in the tree
(has no such use). That who will use so should check. Probably adding
comment will be better (I'll add).
>
>> +
>> + parent= tree->parents;
>> + *parent = &tree->root; element= tree->root;
>> + while (element != &tree->null_element)
>> + {
>> + *++parent= R_ELEMENT_CHILD(element, tree->del_direction);
>> + element= ELEMENT_CHILD(element, tree->del_direction);
>> + }
>> + parent--;
>> + tree->free_element= **parent;
>> + tree_exclude(tree, parent);
>> + tree->elements_in_tree--;
>> + }
>>
>> parent= tree->parents;
>> *parent = &tree->root; element= tree->root;
>> diff --git a/sql/item_sum.cc b/sql/item_sum.cc
>> index 1cfee1a..8bcc2ce 100644
>> --- a/sql/item_sum.cc
>> +++ b/sql/item_sum.cc
>> @@ -3390,7 +3390,7 @@ Item_func_group_concat::fix_fields(THD *thd, Item **ref)
>> fixed= 1;
>> return FALSE;
>> }
>> -
>> +struct tree_node_size {void *left,*right; uint32 f;};
> why do you need that?
>
>>
>> bool Item_func_group_concat::setup(THD *thd)
>> {
>> @@ -3499,17 +3499,50 @@ bool Item_func_group_concat::setup(THD *thd)
>>
>> if (arg_count_order)
>> {
>> + uint min_rec_length= 1; //at least 1 byte per rec (coma)
>> + {
>> + List_iterator_fast<Item> li(all_fields);
>> + Item *item;
>> + while ((item= li++))
>> + {
>> + switch (item->result_type())
>> + {
>> + case STRING_RESULT:
>> + break; // could be empty string
>> + case DECIMAL_RESULT:
>> + case REAL_RESULT:
>> + min_rec_length+=2;
>> + break;
>> + case INT_RESULT:
>> + min_rec_length++;
>> + break;
>> + case TIME_RESULT:
>> + min_rec_length+=6;
>> + break;
>> + default:
>> + case ROW_RESULT:
>> + DBUG_ASSERT(0);
>> + break;
>> + }
>> + }
>> + }
> yeah... that'll work, of course, but for strings (most common case)
> you'll allocates tens or hundreds (if not thousands) times more memory
> than needed :(
>
> A hackish workaround could be to adjust tree->elements_limit (in
> Item_func_group_concat::add) after each insertion. But in this case it
> would be simpler to limit the tree by size (in bytes) and adjust tree
> size after each insertion. What do you think about it?
I think that even for characters there is not direct correspondence
between bytes and number of characters... so it is possible to make
limit by bytes in case we have only strings and put it as <length> *
<maximum bytes per symbol for given charset> (here better to talk to Bar
to ask if there is a pitfalls as difference in client/server/item
charsets (I think should not be)).
Also we have to take into account that we have key representation in the
tree so probably + 1 bytes for null (I am not sure).
It is worse problem (IMHO) is that it will mean constant
allocating/freeing memory (with limit by number of elements tree is kind
of freeze when it reach the limit). I am not sure if it is serious problem.
I do not know more obstacles except above. And Limit by size is already
present in the tree, but it just free all tree and start from beginning
(so should be changed).
>
> Regards,
> Sergei
> Chief Architect MariaDB
> and security@mariadb.org
>
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