mysql - 为什么并行执行查询时 MySQL 的性能会下降？

Question

问题：为什么并行执行时连接几乎空表的查询的 MySQL 性能下降？

以下是对我所面临问题的更详细说明。我在 MySQL 中有两个表

CREATE TABLE first (
    num int(10) NOT NULL,
    UNIQUE KEY key_num (num)
) ENGINE=InnoDB

CREATE TABLE second (
    num int(10) NOT NULL,
    num2 int(10) NOT NULL,
    UNIQUE KEY key_num (num, num2)
) ENGINE=InnoDB

第一个包含大约一千条记录。第二个是空的或包含很少的记录。它还包含与问题有关的双索引：单索引问题就消失了。现在我正在尝试对这些表并行进行许多相同的查询。每个查询如下所示：

SELECT first.num
FROM first
LEFT JOIN second AS second_1 ON second_1.num = -1 # non-existent key
LEFT JOIN second AS second_2 ON second_2.num = -2 # non-existent key
LEFT JOIN second AS second_3 ON second_3.num = -3 # non-existent key
LEFT JOIN second AS second_4 ON second_4.num = -4 # non-existent key
LEFT JOIN second AS second_5 ON second_5.num = -5 # non-existent key
LEFT JOIN second AS second_6 ON second_6.num = -6 # non-existent key
WHERE second_1.num IS NULL
  AND second_2.num IS NULL
  AND second_3.num IS NULL
  AND second_4.num IS NULL
  AND second_5.num IS NULL
  AND second_6.num IS NULL

我遇到的问题是，在 8 核机器上性能几乎没有线性提升，实际上我有下降。即有一个进程，我每秒的典型请求数约为 200。有两个进程而不是预期的每秒增加 300 - 400 个查询，我实际上下降到 150 个。对于 10 个进程，我只有 70 个查询每秒。我用于测试的 Perl 代码如下所示：

#!/usr/bin/perl

use strict;
use warnings;

use DBI;
use Parallel::Benchmark;
use SQL::Abstract;
use SQL::Abstract::Plugin::InsertMulti;

my $children_dbh;

foreach my $second_table_row_count (0, 1, 1000) {
    print '#' x 80, "\nsecond_table_row_count = $second_table_row_count\n";
    create_and_fill_tables(1000, $second_table_row_count);
    foreach my $concurrency (1, 2, 3, 4, 6, 8, 10, 20) {
        my $bm = Parallel::Benchmark->new(
            'benchmark' => sub {
                _run_sql();
                return 1;
            },
            'concurrency' => $concurrency,
            'time' => 3,
        );
        my $result = $bm->run();
    }
}

sub create_and_fill_tables {
    my ($first_table_row_count, $second_table_row_count) = @_;
    my $dbh = dbi_connect();
    {
        $dbh->do(q{DROP TABLE IF EXISTS first});
        $dbh->do(q{
            CREATE TABLE first (
                num int(10) NOT NULL,
                UNIQUE KEY key_num (num)
            ) ENGINE=InnoDB
        });
        if ($first_table_row_count) {
            my ($stmt, @bind) = SQL::Abstract->new()->insert_multi(
                'first',
                ['num'],
                [map {[$_]} 1 .. $first_table_row_count],
            );
            $dbh->do($stmt, undef, @bind);
        }
    }
    {
        $dbh->do(q{DROP TABLE IF EXISTS second});
        $dbh->do(q{
            CREATE TABLE second (
                num int(10) NOT NULL,
                num2 int(10) NOT NULL,
                UNIQUE KEY key_num (num, num2)
            ) ENGINE=InnoDB
        });
        if ($second_table_row_count) {
            my ($stmt, @bind) = SQL::Abstract->new()->insert_multi(
                'second',
                ['num'],
                [map {[$_]} 1 .. $second_table_row_count],
            );
            $dbh->do($stmt, undef, @bind);
        }
    }
}

sub _run_sql {
    $children_dbh ||= dbi_connect();
    $children_dbh->selectall_arrayref(q{
        SELECT first.num
        FROM first
        LEFT JOIN second AS second_1 ON second_1.num = -1
        LEFT JOIN second AS second_2 ON second_2.num = -2
        LEFT JOIN second AS second_3 ON second_3.num = -3
        LEFT JOIN second AS second_4 ON second_4.num = -4
        LEFT JOIN second AS second_5 ON second_5.num = -5
        LEFT JOIN second AS second_6 ON second_6.num = -6
        WHERE second_1.num IS NULL
          AND second_2.num IS NULL
          AND second_3.num IS NULL
          AND second_4.num IS NULL
          AND second_5.num IS NULL
          AND second_6.num IS NULL
    });
}

sub dbi_connect {
    return DBI->connect(
        'dbi:mysql:'
            . 'database=tmp'
            . ';host=localhost'
            . ';port=3306',
        'root',
        '',
    );
}

对于比较像这样的查询并发执行以提高性能：

SELECT first.num
FROM first
LEFT JOIN second AS second_1 ON second_1.num = 1 # existent key
LEFT JOIN second AS second_2 ON second_2.num = 2 # existent key
LEFT JOIN second AS second_3 ON second_3.num = 3 # existent key
LEFT JOIN second AS second_4 ON second_4.num = 4 # existent key
LEFT JOIN second AS second_5 ON second_5.num = 5 # existent key
LEFT JOIN second AS second_6 ON second_6.num = 6 # existent key
WHERE second_1.num IS NOT NULL
  AND second_2.num IS NOT NULL
  AND second_3.num IS NOT NULL
  AND second_4.num IS NOT NULL
  AND second_5.num IS NOT NULL
  AND second_6.num IS NOT NULL

测试结果、cpu 和磁盘使用测量在这里：

* 表 `first` 有 1000 行
* 表 `second` 有 6 行：`[1,1],[2,2],..[6,6]`

查询：
    SELECT first.num
    从第一
    LEFT JOIN second AS second_1 ON second_1.num = -1 # 不存在的键
    LEFT JOIN second AS second_2 ON second_2.num = -2 # 不存在的键
    LEFT JOIN second AS second_3 ON second_3.num = -3 # 不存在的键
    LEFT JOIN second AS second_4 ON second_4.num = -4 # 不存在的键
    LEFT JOIN second AS second_5 ON second_5.num = -5 # 不存在的键
    LEFT JOIN second AS second_6 ON second_6.num = -6 # 不存在的键
    在哪里 second_1.num 为空
      AND second_2.num 为空
      AND second_3.num 为空
      AND second_4.num 为空
      AND second_5.num 为空
      AND second_6.num 为空

结果：
    并发：1，速度：162.910/秒
    并发：2，速度：137.818/秒
    并发：3，速度：130.728/秒
    并发：4，速度：107.387/秒
    并发：6，速度：90.513/秒
    并发：8，速度：80.445/秒
    并发：10，速度：80.381/秒
    并发：20，速度：84.069/秒

在 6 个进程中运行查询的最后 60 分钟后的系统使用情况：
    $ iostat -cdkx 60

    平均 CPU：%user %nice %system %iowait %steal %idle
              74.82 0.00 0.08 0.00 0.08 25.02

    设备：rrqm/s wrqm/sr/sw/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
    sda1 0.00 0.00 0.00 0.12 0.00 0.80 13.71 0.00 1.43 1.43 0.02
    sdf10 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdf4 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 30.00 15.00 0.05
    sdm 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf8 0.00 0.00 0.00 0.37 0.00 1.24 6.77 0.00 5.00 3.18 0.12
    sdf6 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdf9 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 0.00 0.00 0.00
    自卫队 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf3 0.00 0.00 0.00 0.08 0.00 1.33 32.00 0.00 4.00 4.00 0.03
    sdf2 0.00 0.00 0.00 0.17 0.00 1.37 16.50 0.00 3.00 3.00 0.05
    sdf15 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf1 0.00 0.00 0.00 0.05 0.00 0.40 16.00 0.00 0.00 0.00 0.00
    sdf13 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdf5 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 50.00 25.00 0.08
    sdm2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdm1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf12 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdf11 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdf7 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    md0 0.00 0.00 0.00 0.97 0.00 13.95 28.86 0.00 0.00 0.00 0.00

################################################# ##############################

查询：
    SELECT first.num
    从第一
    LEFT JOIN second AS second_1 ON second_1.num = 1 # 存在键
    LEFT JOIN second AS second_2 ON second_2.num = 2 # 存在键
    LEFT JOIN second AS second_3 ON second_3.num = 3 # 存在键
    LEFT JOIN second AS second_4 ON second_4.num = 4 # 存在键
    LEFT JOIN second AS second_5 ON second_5.num = 5 # 存在键
    LEFT JOIN second AS second_6 ON second_6.num = 6 # 存在键
    在哪里 second_1.num 不为空
      AND second_2.num 不为空
      AND second_3.num 不为空
      并且 second_4.num 不为空
      并且 second_5.num 不为空
      并且 second_6.num 不为空

结果：
    并发：1，速度：875.973/秒
    并发：2，速度：944.986/秒
    并发：3，速度：1256.072/秒
    并发：4，速度：1401.657/秒
    并发：6，速度：1354.351/秒
    并发：8，速度：1110.100/秒
    并发：10，速度：1145.251/秒
    并发：20，速度：1142.514/秒

在 6 个进程中运行查询的最后 60 分钟后的系统使用情况：
    $ iostat -cdkx 60

    平均 CPU：%user %nice %system %iowait %steal %idle
              74.40 0.00 0.53 0.00 0.06 25.01

    设备：rrqm/s wrqm/sr/sw/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
    sda1 0.00 0.00 0.00 0.02 0.00 0.13 16.00 0.00 0.00 0.00 0.00
    sdf10 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdf4 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdm 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf8 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdf6 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 0.00 0.00 0.00
    sdf9 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    自卫队 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf3 0.00 0.00 0.00 0.13 0.00 2.67 40.00 0.00 3.75 2.50 0.03
    sdf2 0.00 0.00 0.00 0.23 0.00 2.72 23.29 0.00 2.14 1.43 0.03
    sdf15 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf14 0.00 0.00 0.00 0.98 0.00 0.54 1.10 0.00 2.71 2.71 0.27
    sdf1 0.00 0.00 0.00 0.08 0.00 1.47 35.20 0.00 8.00 6.00 0.05
    sdf13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf5 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    sdm2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdm1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf12 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    sdf11 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 0.00 0.00 0.00
    sdf7 0.00 0.00 0.00 0.03 0.00 1.07 64.00 0.00 10.00 5.00 0.02
    md0 0.00 0.00 0.00 1.70 0.00 15.92 18.74 0.00 0.00 0.00 0.00

################################################# ##############################

这个服务器有很多可用内存。顶部示例：
    top - 19:02:59 up 4:23, 4 users, load average: 4.43, 3.03, 2.01
    任务：共 218 个，运行 1 个，睡眠 217 个，停止 0 个，僵尸 0 个
    CPU：72.8%us、0.7%sy、0.0%ni、26.3%id、0.0%wa、0.0%hi、0.0%si、0.1%st
    内存：总计 71701416k，已使用 22183980k，可用 49517436k，284k 缓冲区
    交换：总共 0k，使用 0k，免费 0k，缓存 1282768k

      PID 用户 PR NI VIRT RES SHR S %CPU %MEM TIME+ 命令
     2506 mysql 20 0 51.7g 17g 5920 S 590 25.8 213:15.12 mysqld
     9348 topadver 20 0 72256 11m 1428 S 2 0.0 0:01.45 perl
     9349 topadver 20 0 72256 11m 1428 S 2 0.0 0:01.44 perl
     9350 topadver 20 0 72256 11m 1428 S 2 0.0 0:01.45 perl
     9351 topadver 20 0 72256 11m 1428 S 1 0.0 0:01.44 perl
     9352 topadver 20 0 72256 11m 1428 S 1 0.0 0:01.44 perl
     9353 topadver 20 0 72256 11m 1428 S 1 0.0 0:01.44 perl
     9346 顶部 20 0 19340 1504 1064 R 0 0.0 0:01.89 顶部

有谁知道为什么使用不存在的键进行查询会降低性能？

score 8 · Accepted Answer

写得很好的问题，显示了一些研究。

出于好奇，我尝试了 MySQL 5.6 来看看那里的工具对这些查询有什么看法。

首先，请注意查询是不同的：

将存在/不存在的关键案例的值从“1”更改为“-1”是一回事
在 WHERE 子句中将“second_1.num IS NOT NULL”更改为“second_1.num IS NULL”是另一个。

使用 EXPLAIN 给出不同的计划：

EXPLAIN SELECT `first`.num
FROM `first`
LEFT JOIN `second` AS second_1 ON second_1.num = -1 # non-existent key
LEFT JOIN `second` AS second_2 ON second_2.num = -2 # non-existent key
LEFT JOIN `second` AS second_3 ON second_3.num = -3 # non-existent key
LEFT JOIN `second` AS second_4 ON second_4.num = -4 # non-existent key
LEFT JOIN `second` AS second_5 ON second_5.num = -5 # non-existent key
LEFT JOIN `second` AS second_6 ON second_6.num = -6 # non-existent key
WHERE second_1.num IS NULL
AND second_2.num IS NULL
AND second_3.num IS NULL
AND second_4.num IS NULL
AND second_5.num IS NULL
AND second_6.num IS NULL
;
id      select_type     table   type    possible_keys   key     key_len ref     rows    Extra
1       SIMPLE  first   index   NULL    key_num 4       NULL    1000    Using index
1       SIMPLE  second_1        ref     key_num key_num 4       const   1       Using where; Not exists; Using index
1       SIMPLE  second_2        ref     key_num key_num 4       const   1       Using where; Not exists; Using index
1       SIMPLE  second_3        ref     key_num key_num 4       const   1       Using where; Not exists; Using index
1       SIMPLE  second_4        ref     key_num key_num 4       const   1       Using where; Not exists; Using index
1       SIMPLE  second_5        ref     key_num key_num 4       const   1       Using where; Not exists; Using index
1       SIMPLE  second_6        ref     key_num key_num 4       const   1       Using where; Not exists; Using index

与

EXPLAIN SELECT `first`.num
FROM `first`
LEFT JOIN `second` AS second_1 ON second_1.num = 1 # existent key
LEFT JOIN `second` AS second_2 ON second_2.num = 2 # existent key
LEFT JOIN `second` AS second_3 ON second_3.num = 3 # existent key
LEFT JOIN `second` AS second_4 ON second_4.num = 4 # existent key
LEFT JOIN `second` AS second_5 ON second_5.num = 5 # existent key
LEFT JOIN `second` AS second_6 ON second_6.num = 6 # existent key
WHERE second_1.num IS NOT NULL
AND second_2.num IS NOT NULL
AND second_3.num IS NOT NULL
AND second_4.num IS NOT NULL
AND second_5.num IS NOT NULL
AND second_6.num IS NOT NULL
;
id      select_type     table   type    possible_keys   key     key_len ref     rows    Extra
1       SIMPLE  second_1        ref     key_num key_num 4       const   1       Using index
1       SIMPLE  second_2        ref     key_num key_num 4       const   1       Using index
1       SIMPLE  second_3        ref     key_num key_num 4       const   1       Using index
1       SIMPLE  second_4        ref     key_num key_num 4       const   1       Using index
1       SIMPLE  second_5        ref     key_num key_num 4       const   1       Using index
1       SIMPLE  second_6        ref     key_num key_num 4       const   1       Using index
1       SIMPLE  first   index   NULL    key_num 4       NULL    1000    Using index; Using join buffer (Block Nested Loop)

使用 JSON 格式，我们有：

EXPLAIN FORMAT=JSON SELECT `first`.num
FROM `first`
LEFT JOIN `second` AS second_1 ON second_1.num = -1 # non-existent key
LEFT JOIN `second` AS second_2 ON second_2.num = -2 # non-existent key
LEFT JOIN `second` AS second_3 ON second_3.num = -3 # non-existent key
LEFT JOIN `second` AS second_4 ON second_4.num = -4 # non-existent key
LEFT JOIN `second` AS second_5 ON second_5.num = -5 # non-existent key
LEFT JOIN `second` AS second_6 ON second_6.num = -6 # non-existent key
WHERE second_1.num IS NULL
AND second_2.num IS NULL
AND second_3.num IS NULL
AND second_4.num IS NULL
AND second_5.num IS NULL
AND second_6.num IS NULL
;
EXPLAIN
{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "first",
          "access_type": "index",
          "key": "key_num",
          "key_length": "4",
          "rows": 1000,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "second_1",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "not_exists": true,
          "using_index": true,
          "attached_condition": "<if>(found_match(second_1), isnull(`test`.`second_1`.`num`), true)"
        }
      },
      {
        "table": {
          "table_name": "second_2",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "not_exists": true,
          "using_index": true,
          "attached_condition": "<if>(found_match(second_2), isnull(`test`.`second_2`.`num`), true)"
        }
      },
      {
        "table": {
          "table_name": "second_3",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "not_exists": true,
          "using_index": true,
          "attached_condition": "<if>(found_match(second_3), isnull(`test`.`second_3`.`num`), true)"
        }
      },
      {
        "table": {
          "table_name": "second_4",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "not_exists": true,
          "using_index": true,
          "attached_condition": "<if>(found_match(second_4), isnull(`test`.`second_4`.`num`), true)"
        }
      },
      {
        "table": {
          "table_name": "second_5",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "not_exists": true,
          "using_index": true,
          "attached_condition": "<if>(found_match(second_5), isnull(`test`.`second_5`.`num`), true)"
        }
      },
      {
        "table": {
          "table_name": "second_6",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "not_exists": true,
          "using_index": true,
          "attached_condition": "<if>(found_match(second_6), isnull(`test`.`second_6`.`num`), true)"
        }
      }
    ]
  }
}

与

EXPLAIN FORMAT=JSON SELECT `first`.num
FROM `first`
LEFT JOIN `second` AS second_1 ON second_1.num = 1 # existent key
LEFT JOIN `second` AS second_2 ON second_2.num = 2 # existent key
LEFT JOIN `second` AS second_3 ON second_3.num = 3 # existent key
LEFT JOIN `second` AS second_4 ON second_4.num = 4 # existent key
LEFT JOIN `second` AS second_5 ON second_5.num = 5 # existent key
LEFT JOIN `second` AS second_6 ON second_6.num = 6 # existent key
WHERE second_1.num IS NOT NULL
AND second_2.num IS NOT NULL
AND second_3.num IS NOT NULL
AND second_4.num IS NOT NULL
AND second_5.num IS NOT NULL
AND second_6.num IS NOT NULL
;
EXPLAIN
{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "second_1",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "second_2",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "second_3",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "second_4",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "second_5",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "second_6",
          "access_type": "ref",
          "possible_keys": [
            "key_num"
          ],
          "key": "key_num",
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "first",
          "access_type": "index",
          "key": "key_num",
          "key_length": "4",
          "rows": 1000,
          "filtered": 100,
          "using_index": true,
          "using_join_buffer": "Block Nested Loop"
        }
      }
    ]
  }
}

查看运行时性能模式检测的表 io，我们有：

truncate table performance_schema.objects_summary_global_by_type;
select * from performance_schema.objects_summary_global_by_type
where OBJECT_NAME in ("first", "second");
OBJECT_TYPE OBJECT_SCHEMA   OBJECT_NAME COUNT_STAR  SUM_TIMER_WAIT  MIN_TIMER_WAIT  AVG_TIMER_WAIT  MAX_TIMER_WAIT
TABLE   test    first   0   0   0   0   0
TABLE   test    second  0   0   0   0   0
SELECT `first`.num
FROM `first`
LEFT JOIN `second` AS second_1 ON second_1.num = -1 # non-existent key
LEFT JOIN `second` AS second_2 ON second_2.num = -2 # non-existent key
LEFT JOIN `second` AS second_3 ON second_3.num = -3 # non-existent key
LEFT JOIN `second` AS second_4 ON second_4.num = -4 # non-existent key
LEFT JOIN `second` AS second_5 ON second_5.num = -5 # non-existent key
LEFT JOIN `second` AS second_6 ON second_6.num = -6 # non-existent key
WHERE second_1.num IS NULL
AND second_2.num IS NULL
AND second_3.num IS NULL
AND second_4.num IS NULL
AND second_5.num IS NULL
AND second_6.num IS NULL
;
(...)
select * from performance_schema.objects_summary_global_by_type
where OBJECT_NAME in ("first", "second");
OBJECT_TYPE OBJECT_SCHEMA   OBJECT_NAME COUNT_STAR  SUM_TIMER_WAIT  MIN_TIMER_WAIT  AVG_TIMER_WAIT  MAX_TIMER_WAIT
TABLE   test    first   1003    5705014442  1026171 5687889 87356557
TABLE   test    second  6012    271786533972    537266  45207298    1123939292

相对于：

select * from performance_schema.objects_summary_global_by_type
where OBJECT_NAME in ("first", "second");
OBJECT_TYPE OBJECT_SCHEMA   OBJECT_NAME COUNT_STAR  SUM_TIMER_WAIT  MIN_TIMER_WAIT  AVG_TIMER_WAIT  MAX_TIMER_WAIT
TABLE   test    first   1003    5211074603  969338  5195454 61066176
TABLE   test    second  24  458656783   510085  19110361    66229860

可扩展的查询在 table 中几乎没有表 IO second。不伸缩的查询在表中做 6K 表 IO second，或表大小的 6 倍first。

这是因为查询计划不同，反过来又因为查询不同（IS NOT NULL 与 IS NULL）。

我认为这回答了与性能相关的问题。

请注意，在我的测试中，这两个查询都返回了 1000 行，这可能不是您想要的。在调整查询以使其更快之前，请确保它按预期工作。

score 1 · Accepted Answer

我建议尝试一种方法，每个人都fork使用自己的连接（在我看来，现在$children_dbh拥有数据库连接的是一个共享变量）。或者，更好的是，实现所谓的connection pool，每个客户端进程将在需要时从中获取连接，并在不再需要时将其“归还”。

检查此答案以获取更多详细信息：给出它的线程是关于 Java 的，但它实际上是关于 MySQL 组织的一些通用原则。这个答案也可能有用。

PS这里描述了有点类似的情况（我认为），并且有关于如何组织连接池的详细说明。

mysql - 为什么并行执行查询时 MySQL 的性能会下降？

2 回答 2

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