特定数据的描述排序性能不好-Postgresql 10.3

我有一个关于一个奇怪的（？）案例的问题，我发现这个案例是关于排序的 Postgresql（具体为10.3）

我有一个包含以下列的表

users

：

• id

  -

  varchar（36）

  -id为UUID格式

• firstname

  -

  varchar（255）

  ,

• lastname

  -

  varchar（255）

将创建以下索引：

create unique index users_pkey on users (id);  
create index user_firstname on users (firstname);  
create index user_lastname on users (lastname);  

现在，让我们考虑每个数据集的两个查询。< /P>

我将~100k行放入表中，其中

firstname

是一个随机的10字符字符串

（1a）

以及此查询的执行计划：

Limit  (cost=7665.06..7665.18 rows=50 width=48) (actual time=105.012..105.016 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=48) (actual time=105.012..105.014 rows=50 loops=1)
        Sort Key: firstname, id
        Sort Method: top-N heapsort  Memory: 31kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=48) (actual time=0.009..21.510 rows=100003 loops=1)
Planning time: 0.066 ms
Execution time: 105.031 ms

Limit  (cost=7665.06..7665.18 rows=50 width=48) (actual time=105.586..105.590 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=48) (actual time=105.586..105.589 rows=50 loops=1)
        Sort Key: firstname DESC, id DESC
        Sort Method: top-N heapsort  Memory: 31kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=48) (actual time=0.010..21.670 rows=100003 loops=1)
Planning time: 0.068 ms
Execution time: 105.606 ms

Limit  (cost=7665.06..7665.18 rows=50 width=43) (actual time=99.572..99.577 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=43) (actual time=99.572..99.573 rows=50 loops=1)
        Sort Key: firstname, id
        Sort Method: top-N heapsort  Memory: 29kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=43) (actual time=0.009..23.660 rows=100003 loops=1)
Planning time: 0.064 ms
Execution time: 99.592 ms

Limit  (cost=7665.06..7665.18 rows=50 width=43) (actual time=659.786..659.791 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=43) (actual time=659.785..659.786 rows=50 loops=1)
        Sort Key: firstname DESC, id DESC
        Sort Method: top-N heapsort  Memory: 32kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=43) (actual time=0.010..21.510 rows=100003 loops=1)
Planning time: 0.066 ms
Execution time: 659.804 ms

1b）

排序已更改-desc而不是asc

以及此查询的执行计划：

Limit  (cost=7665.06..7665.18 rows=50 width=48) (actual time=105.012..105.016 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=48) (actual time=105.012..105.014 rows=50 loops=1)
        Sort Key: firstname, id
        Sort Method: top-N heapsort  Memory: 31kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=48) (actual time=0.009..21.510 rows=100003 loops=1)
Planning time: 0.066 ms
Execution time: 105.031 ms

Limit  (cost=7665.06..7665.18 rows=50 width=48) (actual time=105.586..105.590 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=48) (actual time=105.586..105.589 rows=50 loops=1)
        Sort Key: firstname DESC, id DESC
        Sort Method: top-N heapsort  Memory: 31kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=48) (actual time=0.010..21.670 rows=100003 loops=1)
Planning time: 0.068 ms
Execution time: 105.606 ms

Limit  (cost=7665.06..7665.18 rows=50 width=43) (actual time=99.572..99.577 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=43) (actual time=99.572..99.573 rows=50 loops=1)
        Sort Key: firstname, id
        Sort Method: top-N heapsort  Memory: 29kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=43) (actual time=0.009..23.660 rows=100003 loops=1)
Planning time: 0.064 ms
Execution time: 99.592 ms

Limit  (cost=7665.06..7665.18 rows=50 width=43) (actual time=659.786..659.791 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=43) (actual time=659.785..659.786 rows=50 loops=1)
        Sort Key: firstname DESC, id DESC
        Sort Method: top-N heapsort  Memory: 32kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=43) (actual time=0.010..21.510 rows=100003 loops=1)
Planning time: 0.066 ms
Execution time: 659.804 ms

到目前为止，一切顺利。在两个方向上进行排序所需的时间相似

我们考虑第二个数据集。我将~100k行放入表中，其中

firstname

是以下格式的字符串：johnxxx，其中XXXXX是数字序列，即John00000、John00001、John00002、John00003、…、John99998、John99999

2a）

以及此查询的执行计划：

Limit  (cost=7665.06..7665.18 rows=50 width=48) (actual time=105.012..105.016 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=48) (actual time=105.012..105.014 rows=50 loops=1)
        Sort Key: firstname, id
        Sort Method: top-N heapsort  Memory: 31kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=48) (actual time=0.009..21.510 rows=100003 loops=1)
Planning time: 0.066 ms
Execution time: 105.031 ms

Limit  (cost=7665.06..7665.18 rows=50 width=48) (actual time=105.586..105.590 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=48) (actual time=105.586..105.589 rows=50 loops=1)
        Sort Key: firstname DESC, id DESC
        Sort Method: top-N heapsort  Memory: 31kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=48) (actual time=0.010..21.670 rows=100003 loops=1)
Planning time: 0.068 ms
Execution time: 105.606 ms

Limit  (cost=7665.06..7665.18 rows=50 width=43) (actual time=99.572..99.577 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=43) (actual time=99.572..99.573 rows=50 loops=1)
        Sort Key: firstname, id
        Sort Method: top-N heapsort  Memory: 29kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=43) (actual time=0.009..23.660 rows=100003 loops=1)
Planning time: 0.064 ms
Execution time: 99.592 ms

Limit  (cost=7665.06..7665.18 rows=50 width=43) (actual time=659.786..659.791 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=43) (actual time=659.785..659.786 rows=50 loops=1)
        Sort Key: firstname DESC, id DESC
        Sort Method: top-N heapsort  Memory: 32kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=43) (actual time=0.010..21.510 rows=100003 loops=1)
Planning time: 0.066 ms
Execution time: 659.804 ms

2b）

排序已更改-desc而不是asc

以及此查询的执行计划：

Limit  (cost=7665.06..7665.18 rows=50 width=48) (actual time=105.012..105.016 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=48) (actual time=105.012..105.014 rows=50 loops=1)
        Sort Key: firstname, id
        Sort Method: top-N heapsort  Memory: 31kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=48) (actual time=0.009..21.510 rows=100003 loops=1)
Planning time: 0.066 ms
Execution time: 105.031 ms

Limit  (cost=7665.06..7665.18 rows=50 width=48) (actual time=105.586..105.590 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=48) (actual time=105.586..105.589 rows=50 loops=1)
        Sort Key: firstname DESC, id DESC
        Sort Method: top-N heapsort  Memory: 31kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=48) (actual time=0.010..21.670 rows=100003 loops=1)
Planning time: 0.068 ms
Execution time: 105.606 ms

Limit  (cost=7665.06..7665.18 rows=50 width=43) (actual time=99.572..99.577 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=43) (actual time=99.572..99.573 rows=50 loops=1)
        Sort Key: firstname, id
        Sort Method: top-N heapsort  Memory: 29kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=43) (actual time=0.009..23.660 rows=100003 loops=1)
Planning time: 0.064 ms
Execution time: 99.592 ms

Limit  (cost=7665.06..7665.18 rows=50 width=43) (actual time=659.786..659.791 rows=50 loops=1)
  ->  Sort  (cost=7665.06..7915.07 rows=100003 width=43) (actual time=659.785..659.786 rows=50 loops=1)
        Sort Key: firstname DESC, id DESC
        Sort Method: top-N heapsort  Memory: 32kB
        ->  Seq Scan on users  (cost=0.00..4343.03 rows=100003 width=43) (actual time=0.010..21.510 rows=100003 loops=1)
Planning time: 0.066 ms
Execution time: 659.804 ms

对于第二个数据集，第二次查询（

2b

）的速度要慢7倍

概括地说：

+----------------+------------+------------+
| Query\Data set |     1      |      2     |
+----------------+------------+------------+
|  1             | 105.031 ms | 99.592 ms  |
|  2             | 105.606 ms | 659.804 ms |
+----------------+------------+------------+

最后，我的问题。

---

为什么对第二个数据集的第二次查询比其他情况慢6-7倍？

在添加额外的50k数据后，您是否重建了索引？检查碎片。

在添加额外的50k数据后，是否重新构建了索引？检查碎片。

重新生成索引。这些问题发生在具有新数据的新表上，因此我的假设是碎片问题不是这种情况，但可以肯定的是，我如何检查碎片？转到表->索引，右键单击索引，选择属性，然后在选项卡的左侧菜单上选择碎片。您使用的是什么postgresql客户端？我在pgAdmin和Intellij中都找不到这个选项。我最真诚的道歉，我以为你在使用SSMS。关于这一点，我的同事发现这篇文章特别有用：（）重建索引。这些问题发生在具有新数据的新表上，因此我的假设是碎片问题不是这种情况，但可以肯定的是，我如何检查碎片？转到表->索引，右键单击索引，选择属性，然后在选项卡的左侧菜单上选择碎片。您使用的是什么postgresql客户端？我在pgAdmin和Intellij中都找不到这个选项。我最真诚的道歉，我以为你在使用SSMS。关于这一点，我的同事们发现这篇文章特别有用：（）