Re: bzr commit into mysql-trunk-bugfixing branch (tor.didriksen:3250) WL#1393

[Jorgen Loland]

On 11/29/2010 12:21 PM, Tor Didriksen wrote:
> #At file:///export/home/didrik/repo/next-mr-opt-team-wl1393-merge/ based on
> revid:tor.didriksen@stripped
>
>   3250 Tor Didriksen	2010-11-29
>        WL#1393 Optimizing filesort with small limit
>
>        Many web customers have to do
>        "SELECT ... ORDER BY non_index_column LIMIT X",
>
>        When X *<Row Size>  is smaller than sort_buff_size we can use
>        the following algoritm to speed up the sort:
>
>        - Create a queue to hold 'limit' keys.
>        - Scan through the table and store the first (last if DESC) keys in the queue
>        - Return values from queue

Bug found:

   if (param.max_rows != HA_POS_ERROR &&
       check_if_pq_applicable(&param, &table_sort,
                              table, num_rows, memory_available))
   {
     DBUG_PRINT("info", ("filesort PQ is applicable"));
     const size_t compare_length= param.sort_length;
     if (pq.init(param.max_rows,
                 true,                           // max_at_top
                 NULL,                           // compare_function
                 compare_length,
                 &make_sortkey, &param, table_sort.sort_keys))
     {
       // If failed to init pq, fall back to merge-sort.
       DBUG_PRINT("info", ("failed to allocate PQ, fallback to sort-merge"));
       my_free(table_sort.sort_keys);
       table_sort.sort_keys= NULL;
     }
   }

No test case to reproduce this, but if you make pq.init() return 1, 
table_sort.sort_keys is set to NULL and is never reinitialized. Thus, we end up 
with a SEGFAULT once we try to use it:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff0eeb710 (LWP 3848)]
0x000000000076531d in find_all_keys (param=0x7ffff0ee7fc0, select=0x19f3140,
     sort_keys=0x0, buffpek_pointers=0x7ffff0ee8170, tempfile=0x7ffff0ee82d0,
     pq=0x0, found_rows=0x7ffff0ee8508)
     at 
/home/jl208045/mysql/wl1393-review/next-mr-opt-team-wl1393-merge/sql/filesort.cc:674
674             make_sortkey(param, sort_keys[idx++], ref_pos);

-- 
Jørgen Løland | Senior Software Engineer | [phone]
Oracle MySQL
Trondheim, Norway