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问题:为什么并行执行连接几乎空表的查询的MySQL性能下降?
以下是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
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
#!/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',
'',
);
}
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
最佳答案
写得很好的问题,表明了一些研究。
出于好奇,我尝试使用MySQL 5.6来查看那里的工具对这些查询的评价。
首先,请注意查询是不同的:
将存在/不存在的值从“ 1”更改为“ -1”
关键案例是一回事
将“ second_1.num不为空”更改为
WHERE子句中的“ second_1.num IS NULL”是另一个。
使用EXPLAIN会给出不同的计划:
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
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)
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)"
}
}
]
}
}
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"
}
}
]
}
}
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
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
second
中几乎没有表IO。
second
中执行6K表IO,或者是表
first
大小的6倍。
关于mysql - 为什么并行执行查询时MySQL的性能会下降?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/11160522/
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