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周梦康 发表于 2019-02-15 652 次浏览 标签 : Mysql

背景

接着 https://mengkang.net/1328.html 的案例,我们继续磕。
上一篇 GDB 调试 Mysql 实战(三)优先队列排序算法探究(上) 分析了实验3中的row_size为什么是24。其他实验的row_size都是36,扫描行数也不符合预期。这篇就来探究下。

以实验1为例来分析

select `aid`,sum(`pv`) as num from article_rank force index(idx_day_aid_pv)  where `day`>20190115 group by aid order by num desc LIMIT 10;

optimizer_trace.join_execution.steps的结果如下

{
  "join_execution": {
    "select#": 1,
    "steps": [
      {
        "creating_tmp_table": {
          "tmp_table_info": {
            "table": "intermediate_tmp_table",
            "row_length": 20,
            "key_length": 4,
            "unique_constraint": false,
            "location": "memory (heap)",
            "row_limit_estimate": 838860
          }
        }
      },
      {
        "converting_tmp_table_to_ondisk": {
          "cause": "memory_table_size_exceeded",
          "tmp_table_info": {
            "table": "intermediate_tmp_table",
            "row_length": 20,
            "key_length": 4,
            "unique_constraint": false,
            "location": "disk (InnoDB)",
            "record_format": "fixed"
          }
        }
      },
      {
        "filesort_information": [
          {
            "direction": "desc",
            "table": "intermediate_tmp_table",
            "field": "num"
          }
        ],
        "filesort_priority_queue_optimization": {
          "limit": 10,
          "rows_estimate": 1057,
          "row_size": 36,
          "memory_available": 262144,
          "chosen": true
        },
        "filesort_execution": [
        ],
        "filesort_summary": {
          "rows": 11,
          "examined_rows": 649091,
          "number_of_tmp_files": 0,
          "sort_buffer_size": 488,
          "sort_mode": "<sort_key, additional_fields>"
        }
      }
    ]
  }
}

row_size 为什么是 36

(gdb) b Sort_param::init_for_filesort
Breakpoint 1 at 0xf1a89f: file /root/newdb/mysql-server/sql/filesort.cc, line 107.

(gdb) b Filesort::get_addon_fields
Breakpoint 2 at 0xf21231: file /root/newdb/mysql-server/sql/filesort.cc, line 2459.
(gdb) b /root/newdb/mysql-server/sql/filesort.cc:2496
Breakpoint 3 at 0xf212f9: file /root/newdb/mysql-server/sql/filesort.cc, line 2496.
(gdb) b /root/newdb/mysql-server/sql/filesort.cc:2523
Breakpoint 4 at 0xf2145f: file /root/newdb/mysql-server/sql/filesort.cc, line 2523.

排序字段还是实验3一样是16字节,后面20字节则是两个字段相加20字节+ (null_fields + 7) / 8 一个可为空的字段,所以最后是36了。

rows_estimate 为什么是 1057

(gdb) b /root/newdb/mysql-server/sql/filesort.cc:320
Breakpoint 5 at 0xf1b1d9: file /root/newdb/mysql-server/sql/filesort.cc, line 320.
...
Breakpoint 5, filesort (thd=0x7f0214000d80, filesort=0x7f021401f668, sort_positions=false, examined_rows=0x7f022804d050,
    found_rows=0x7f022804d048, returned_rows=0x7f022804d040) at /root/newdb/mysql-server/sql/filesort.cc:320
320      num_rows= table->file->estimate_rows_upper_bound();
(gdb) s
ha_innobase::estimate_rows_upper_bound (this=0x7f0214022b50)
    at /root/newdb/mysql-server/storage/innobase/handler/ha_innodb.cc:13655
ha_innobase::estimate_rows_upper_bound (this=0x7f0214022b50)
    at /root/newdb/mysql-server/storage/innobase/handler/ha_innodb.cc:13655
warning: Source file is more recent than executable.
13655        DBUG_ENTER("estimate_rows_upper_bound");
(gdb) n
13661        update_thd(ha_thd());
(gdb) n
13663        TrxInInnoDB    trx_in_innodb(m_prebuilt->trx);
(gdb) n
13665        m_prebuilt->trx->op_info = "calculating upper bound for table rows";
(gdb) n
13667        index = dict_table_get_first_index(m_prebuilt->table);
(gdb) n
13669        ulint    stat_n_leaf_pages = index->stat_n_leaf_pages;
(gdb) p stat_n_leaf_pages
$19 = 139646902217632
(gdb) n
13671        ut_a(stat_n_leaf_pages > 0);
(gdb) p UNIV_PAGE_SIZE
No symbol "UNIV_PAGE_SIZE" in current context.
(gdb) n
13674            ((ulonglong) stat_n_leaf_pages) * UNIV_PAGE_SIZE;
(gdb) n
13681        estimate = 2 * local_data_file_length
(gdb) p local_data_file_length
$20 = 16384
(gdb) p stat_n_leaf_pages
$21 = 1
(gdb) n
13682            / dict_index_calc_min_rec_len(index);
(gdb) n
13684        m_prebuilt->trx->op_info = "";
(gdb) p estimate
$22 = 1057
(gdb) p dict_index_calc_min_rec_len(index)
$23 = 31

也就是说local_data_file_length是16字节,为当前系统一个内存页大小。
dict_index_calc_min_rec_len注释中写道Calculates the minimum record length in an index.
上面gdb调试记录(文字非截图)中dict_index_calc_min_rec_len(index)的值为31。

ut_a(stat_n_leaf_pages > 0);

local_data_file_length =
    ((ulonglong) stat_n_leaf_pages) * UNIV_PAGE_SIZE;

/* Calculate a minimum length for a clustered index record and from
that an upper bound for the number of rows. Since we only calculate
new statistics in row0mysql.cc when a table has grown by a threshold
factor, we must add a safety factor 2 in front of the formula below. */

estimate = 2 * local_data_file_length
    / dict_index_calc_min_rec_len(index);

(2*16*1024)/31 = 1057,那么为什么dict_index_calc_min_rec_len是31呢?

继续查看源码发现 31 是这么计算出来的,就算知道了31,但是我也还是木有弄懂,为什么扫描行数是 (2*页内存大小)/索引最小行记录长度

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