DynamoDB-如果哈希（或哈希和范围组合）不存在，则放置项目 [英] DynamoDB - Put item if hash (or hash and range combination) doesn't exist

问题描述

这是我的用例：我有一个带有哈希+范围键的Dynamo表。当我在表中放置新项目时，我想进行一次唯一性检查。有时我想保证哈希是唯一的（忽略范围）。其他时候我想允许重复的哈希，但是要保证哈希和范围组合是唯一的。我该怎么做？

我尝试了attribute_not_exists。似乎可以处理第二种情况，即检查哈希+组合键。这是一个PHP示例：

  $ client-&put; putItem（array（
' TableName'=>'test'，
'Item'=> array（
'hash'=> array（'S'=>'abcdefg'），
'range '=> array（'S'=>'一些其他值'），
'无论'=> array（'N'=> 233）
），
' ConditionExpression'=>'attribute_not_exists（hash）'
））; 
  

奇怪的是，我使用 attribute_not_exists（hash）似乎并不重要）或 attribute_not_exists（范围）。他们似乎都做着完全相同的事情。

任何想法如何处理我只想检查 hash 的情况

解决方案

不能。 DynamoDB中的所有项目都按其散列或散列 + 范围（取决于您的表）。

到目前为止的情况的摘要：

• 单个哈希键可以具有多个范围键。


• 每个项目都具有 hash 和范围键


• 您正在发出 PutItem 请求，并必须同时提供哈希和范围


• 在散列或<上提供 ConditionExpression 和 attribute_not_exists code> range 属性名称


• attribute_not_exists 条件仅是检查是否具有该属性名称存在，它不在乎值

我们来看一个示例。让我们从 hash + range 键表开始，并获取以下数据：

1. hash = A，range = 1


2. hash = A，range = 2

有四种可能的情况：

1. 如果尝试放置带有 hash = A，range = 3 和 attribute_not_exists（hash）， PutItem 将成功，因为 attribute_not_exists（hash）计算为 true 。项 hash = A，range = 3 的键满足 attribute_not_exists（hash）的条件。




2. 如果尝试放置带有 hash = A，range = 3 和 attribute_not_exists（范围）， PutItem 将成功，因为 attribute_not_exists（range）计算结果为 true 。项 hash = A，range = 3 满足 attribute_not_exists（range）条件的项不存在。




3. 如果您尝试放置带有 hash = A的项目，范围= 1 和 attribute_not_exists（哈希）， PutItem 将失败，因为 attribute_not_exists（hash）计算为 false 。存在具有键 hash = A，range = 1 的项目，该项目不满足 attribute_not_exists（hash）的条件。




4. 如果您尝试放置带有 hash = A，range = 1 和<$ c的项目$ c> attribute_not_exists（range）， PutItem 将失败，因为 attribute_not_exists（range）计算结果为 false 。存在具有键 hash = A，range = 1 的项目，该项目不满足 attribute_not_exists（range）的条件。

这意味着将发生以下两种情况之一：

1. 数据库中存在 hash + range 对。
   
   
   
   
   • attribute_not_exists（hash）必须为 true
   
   
   • attribute_not_exists（range）必须为 true
   
   


2. 哈希 + 范围对数据库中不存在。
   
   
   
   
   • attribute_not_exists（hash）必须为 false
   
   
   • attribute_not_exists（range）必须为 false
   
   

在两种情况下，无论将其放在哈希表还是范围键。 hash + range 键在整个表中标识单个项目，并且您的条件正在对该项目进行评估。

如果某项具有哈希值 + <$ c，则表示您正在有效地执行放入该项$ c> range 键不存在。

Here are my use cases: I have a Dynamo table with a hash + range key. When I put new items in the table, I want to do a uniqueness check. Sometimes I want to guarantee that the hash is unique (ignoring the range). Other times I want to allow duplicate hashes, but guarantee that the hash and range combination is unique. How can I accomplish this?

I experimented with attribute_not_exists. It seems to handle the second case, where it checks the hash + key combination. Here's a PHP sample:

$client->putItem(array(
    'TableName' => 'test',
    'Item' => array(
        'hash' => array('S' => 'abcdefg'),
        'range' => array('S' => 'some other value'),
        'whatever' => array('N' => 233)
    ),
    'ConditionExpression' => 'attribute_not_exists(hash)'
));

Oddly, it doesn't seem to matter if I use attribute_not_exists(hash) or attribute_not_exists(range). They both seem to do exactly the same thing. Is this how it's supposed to work?

Any idea how to handle the case where I only want to check hash for uniqueness?

解决方案

You can't. All items in DynamoDB are indexed by either their hash or hash+range (depending on your table).

A sort of summary of what is going on so far:

• A single hash key can have multiple range keys.
• Every item has both a hash and a range key
• You are making a PutItem request and must provide both the hash and range
• You are providing a ConditionExpression with attribute_not_exists on either the hash or range attribute name
• The attribute_not_exists condition is merely checking if an attribute with that name exists, it doesn't care about the value

Let's walk through an example. Let's start with a hash+range key table with this data:

1. hash=A,range=1
2. hash=A,range=2

There are four possible cases:

1. If you try to put an item with hash=A,range=3 and attribute_not_exists(hash), the PutItem will succeed because attribute_not_exists(hash) evaluates to true. No item exists with key hash=A,range=3 that satisfies the condition of attribute_not_exists(hash).

2. If you try to put an item with hash=A,range=3 and attribute_not_exists(range), the PutItem will succeed because attribute_not_exists(range) evaluates to true. No item exists with key hash=A,range=3 that satisfies the condition of attribute_not_exists(range).

3. If you try to put an item with hash=A,range=1 and attribute_not_exists(hash), the PutItem will fail because attribute_not_exists(hash) evaluates to false. An item exists with key hash=A,range=1 that does not satisfy the condition of attribute_not_exists(hash).

4. If you try to put an item with hash=A,range=1 and attribute_not_exists(range), the PutItem will fail because attribute_not_exists(range) evaluates to false. An item exists with key hash=A,range=1 that does not satisfy the condition of attribute_not_exists(range).

This means that one of two things will happen:

1. The hash+range pair exists in the database.
   • attribute_not_exists(hash) must be true
   • attribute_not_exists(range) must be true
2. The hash+range pair does not exist in the database.
   • attribute_not_exists(hash) must be false
   • attribute_not_exists(range) must be false

In both cases, you get the same result regardless of whether you put it on the hash or the range key. The hash+range key identifies a single item in the entire table, and your condition is being evaluated on that item.

You are effectively performing a "put this item if an item with this hash+range key does not already exist".

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