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102 lines
4.3 KiB
Markdown
102 lines
4.3 KiB
Markdown
title: Indexes
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summary: Add Indexes to your Data Model to optimize database queries.
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# Indexes
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Indexes are a great way to improve performance in your application, especially as it grows. By adding indexes to your
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data model you can reduce the time taken for the framework to find and filter data objects.
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The addition of an indexes should be carefully evaluated as they can also increase the cost of other operations such as
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`UPDATE`/`INSERT` and `DELETE`. An index on a column whose data is non unique will actually cost you performance.
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E.g. In most cases an index on `boolean` status flag, or `ENUM` state will not increase query performance.
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It's important to find the right balance to achieve fast queries using the optimal set of indexes; For SilverStripe
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applications it's a good practice to:
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- add indexes on columns which are frequently used in `filter`, `where` or `orderBy` statements
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- for these, only include indexes for columns which are the most restrictive (return the least number of rows)
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The SilverStripe framework already places certain indexes for you by default:
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- The primary key for each model has a `PRIMARY KEY` unique index
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- The `ClassName` column if your model inherits from `DataObject`
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- All relationships defined in the model have indexes for their `has_one` entity (for `many_many` relationships
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this index is present on the associative entity).
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## Defining an index
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Indexes are represented on a `DataObject` through the `DataObject::$indexes` array which maps index names to a
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descriptor. There are several supported notations:
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```php
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use SilverStripe\ORM\DataObject;
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class MyObject extends DataObject
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{
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private static $indexes = [
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'<column-name>' => true,
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'<index-name>' => [
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'type' => '<type>',
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'columns' => ['<column-name>', '<other-column-name>'],
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],
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'<index-name>' => ['<column-name>', '<other-column-name>'],
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];
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}
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```
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The `<column-name>` is used to put a standard non-unique index on the column specified. For complex or large tables
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we recommend building the index to suite the requirements of your data.
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The `<index-name>` can be an arbitrary identifier in order to allow for more than one index on a specific database
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column. The "advanced" notation supports more `<type>` notations. These vary between database drivers, but all of them
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support the following:
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* `index`: Standard non unique index.
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* `unique`: Index plus uniqueness constraint on the value
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* `fulltext`: Fulltext content index
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**mysite/code/MyTestObject.php**
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```php
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use SilverStripe\ORM\DataObject;
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class MyTestObject extends DataObject
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{
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private static $db = [
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'MyField' => 'Varchar',
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'MyOtherField' => 'Varchar',
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];
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private static $indexes = [
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'MyIndexName' => ['MyField', 'MyOtherField'],
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];
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}
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```
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<div class="alert" markdown="1">
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Please note that if you have previously used the removed `value` key to define an index's contents, SilverStripe will
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now throw an error. Use `columns` instead.
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</div>
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## Complex/Composite Indexes
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For complex queries it may be necessary to define a complex or composite index on the supporting object. To create a
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composite index, define the fields in the index order as a comma separated list.
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*Note* Most databases only use the leftmost prefix to optimise the query, try to ensure the order of the index and your
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query parameters are the same. e.g.
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- index (col1) - `WHERE col1 = ?`
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- index (col1, col2) = `WHERE (col1 = ? AND col2 = ?)`
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- index (col1, col2, col3) = `WHERE (col1 = ? AND col2 = ? AND col3 = ?)`
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The index would not be used for a query `WHERE col2 = ?` or for `WHERE col1 = ? OR col2 = ?`
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As an alternative to a composite index, you can also create a hashed column which is a combination of information from
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other columns. If this is indexed, smaller and reasonably unique it might be faster that an index on the whole column.
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## Index Creation/Destruction
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Indexes are generated and removed automatically during a `dev/build`. Caution if you're working with large tables and
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modify an index as the next `dev/build` will `DROP` the index, and then `ADD` it.
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As of 3.7.0 `default_sort` fields will automatically become database indexes as this provides significant performance
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benefits.
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## API Documentation
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* [DataObject](api:SilverStripe\ORM\DataObject)
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