91 lines
3.5 KiB
Markdown
91 lines
3.5 KiB
Markdown
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---
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stage: Data Stores
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group: Database
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info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
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---
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# Serializing Data
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**Summary:** don't store serialized data in the database, use separate columns
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and/or tables instead. This includes storing of comma separated values as a
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string.
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Rails makes it possible to store serialized data in JSON, YAML or other formats.
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Such a field can be defined as follows:
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```ruby
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class Issue < ActiveRecord::Model
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serialize :custom_fields
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end
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```
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While it may be tempting to store serialized data in the database there are many
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problems with this. This document outlines these problems and provide an
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alternative.
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## Serialized Data Is Less Powerful
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When using a relational database you have the ability to query individual
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fields, change the schema, index data, and so forth. When you use serialized data
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all of that becomes either very difficult or downright impossible. While
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PostgreSQL does offer the ability to query JSON fields it is mostly meant for
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very specialized use cases, and not for more general use. If you use YAML in
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turn there's no way to query the data at all.
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## Waste Of Space
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Storing serialized data such as JSON or YAML ends up wasting a lot of space.
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This is because these formats often include additional characters (for example, double
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quotes or newlines) besides the data that you are storing.
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## Difficult To Manage
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There comes a time where you must add a new field to the serialized
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data, or change an existing one. Using serialized data this becomes difficult
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and very time consuming as the only way of doing so is to re-write all the
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stored values. To do so you would have to:
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1. Retrieve the data
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1. Parse it into a Ruby structure
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1. Mutate it
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1. Serialize it back to a String
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1. Store it in the database
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On the other hand, if one were to use regular columns adding a column would be:
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```sql
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ALTER TABLE table_name ADD COLUMN column_name type;
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```
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Such a query would take very little to no time and would immediately apply to
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all rows, without having to re-write large JSON or YAML structures.
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Finally, there comes a time when the JSON or YAML structure is no longer
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sufficient and you must migrate away from it. When storing only a few rows
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this may not be a problem, but when storing millions of rows such a migration
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can take hours or even days to complete.
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## Relational Databases Are Not Document Stores
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When storing data as JSON or YAML you're essentially using your database as if
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it were a document store (for example, MongoDB), except you're not using any of the
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powerful features provided by a typical RDBMS _nor_ are you using any of the
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features provided by a typical document store (for example, the ability to index fields
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of documents with variable fields). In other words, it's a waste.
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## Consistent Fields
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One argument sometimes made in favour of serialized data is having to store
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widely varying fields and values. Sometimes this is truly the case, and then
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perhaps it might make sense to use serialized data. However, in 99% of the cases
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the fields and types stored tend to be the same for every row. Even if there is
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a slight difference you can still use separate columns and just not set the ones
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you don't need.
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## The Solution
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The solution is to use separate columns and/or separate tables.
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This allows you to use all the features provided by your database, it
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makes it easier to manage and migrate the data, you conserve space, you can
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index the data efficiently and so forth.
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