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102 lines
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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">DynamoDB</h1><h2 id="data-structure">Data structure</h2><h3 id="non-relational-tables">Non-relational tables</h3><p>DynamoDB is “NoSQL” because it does not support <span><code class="inline-tag">#SQL</code></span> queries and is non-relational meaning there cannot be JOIN operations via <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Creating views with foreign keys"><a href="Foreign_keys_in_SQL.html">foreign_keys</a></span></span></p><p><img src="/static/dynamodb.png" /></p><h3 id="primary-key">Primary key</h3><p>Although the data is stored as a table, one of the attributes is a primary key and the rest of the attributes are effectively the “value” associated with it.</p><p>Because DynamoDB is schemaless, other than the primary key, neither the attributes or their data types need to be defined beforehand and each item can have its own distinct attributes.</p><p>Each item in the table is uniquely identifiable by its primary key.</p><p>There are two types of primary key available:</p><ul><li><p>partition key: a simple primary key composed of one attribute only. Because the primary key is hash-mapped items can be retrieved very rapidly using the primary key. This would be the <code>personId</code> alone.</p></li><li><p>composite key: this comprises a partition key and a <em>sort key</em> both of which are attributes. In a table that has a partition key and a sort key, it’s possible for multiple items to have the same partition key value. However, those items must have different sort key values. You could then query by either key or both. For instance using the <code>PersonId</code> along with <code>LastName</code></p></li></ul><h3 id="secondary-index">Secondary index</h3><p>As well as the index provided by the primary key, you can set one or more <strong>secondary indices</strong>. A secondary index lets you query the data in the table using an alternate key.</p><p>A <strong>global secondary index</strong> is useful for querying data that needs to be accessed using non-primary key attributes. For example, if you have a Users table with <code>UserID</code> as the primary key but often need to fetch users by their <code>Email</code>, a GSI on <code>Email</code> would be appropriate.</p><p>There are also <strong>local secondary indices</strong> but I don’t understand the difference.</p><h2 id="real-example">Real example</h2><p>Below is a specification of the DynamoDB table I am using for my time-entries project:</p><pre><code class="json language-json">{
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"TableName": "TimeEntries",
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"KeyAttributes": {
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"PartitionKey": {
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"AttributeName": "activity_start_end",
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"AttributeType": "S"
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}
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},
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"NonKeyAttributes": [
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{
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"AttributeName": "activity_type",
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"AttributeType": "S"
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},
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{
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"AttributeName": "start",
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"AttributeType": "S"
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},
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{
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"AttributeName": "end",
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"AttributeType": "S"
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},
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{
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"AttributeName": "duration",
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"AttributeType": "N"
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},
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{
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"AttributeName": "description",
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"AttributeType": "S"
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},
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{
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"AttributeName": "year",
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"AttributeType": "S"
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}
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],
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"GlobalSecondaryIndexes": [
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{
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"IndexName": "YearIndex",
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"KeyAttributes": {
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"PartitionKey": {
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"AttributeName": "year",
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"AttributeType": "S"
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},
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"SortKey": {
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"AttributeName": "start",
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"AttributeType": "S"
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}
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},
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"Projection": {
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"ProjectionType": "ALL"
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}
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}</code></pre><p>This defines the attribute <code>activity_start_end</code> as the primary key. This string (<code>S</code>) value is a concatenation of three attributes, which is a way of ensuring each entry for the attribute will be unique.</p><p>The <code>NonKeyAttributes</code> are all the other attributes in addition to the primary key. As mentioned these do not actually need to be defined when setting up the table but they are listed here for clarity.</p><p>I have also defined a GSI. This is derived from the <code>Year</code> attribute. This will group all the items by their <code>Year</code>, allowing me to query directly by year but also helping to chunk the entries which will make look-ups quicker and less expensive.</p><h2 id="related-notes">Related notes</h2></div></article><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">AWS</span><span class="ui basic label zettel-tag" title="Tag">SQL</span><span class="ui basic label zettel-tag" title="Tag">databases</span><span class="ui basic label zettel-tag" title="Tag">dynamodb</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html>
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