Index documentation

- updating index documentation to give a better description of how to improve performance with silverstripe applications

docs/en/02_Developer_Guides/00_Model/12_Indexes.md

@@ -2,10 +2,26 @@ title: Indexes
 summary: Add Indexes to your Data Model to optimize database queries.
 
 # Indexes
+Indexes are a great way to improve performance in your application, especially as it grows. By adding indexes to your 
+data model you can reduce the time taken for the framework to find and filter data objects. 
 
-It is sometimes desirable to add indexes to your data model, whether to optimize queries or add a uniqueness constraint 
-to a field. This is done through the `DataObject::$indexes` map, which maps index names to descriptor arrays that 
-represent each index. There're several supported notations:
+The addition of an indexes should be carefully evaluated as they can also increase the cost of other operations such as 
+`UPDATE`/`INSERT` and `DELETE`. An index which has the same cardinality as the table will actually cost you performance. 
+
+It's important to find the right balance to achieve fast queries using the optimal set of indexes; For SilverStripe 
+applications it's a good practice to: 
+- add indexes on columns which are frequently used in `filter`, `where` or `orderBy` statements
+- for these, only include indexes for columns which are the most restrictive (return the least number of rows)
+
+The SilverStripe framework already places certain indexes for you by default:
+- The primary key for each model has a `PRIMARY KEY` unique index
+- The `ClassName` column if your model is a direct decedent from `DataObject`
+- All relationships defined in the model have indexes for their `has_one` entity (for `many_many` relationships 
+this index is present on the associative entity).
+
+## Defining an index
+Indexes are represented on a data object through the `DataObject::$indexes` array which maps index names to a 
+descriptor. There are several supported notations:
 
 	:::php
 	<?php
@@ -14,22 +30,25 @@ represent each index. There're several supported notations:
 
 		private static $indexes = array(
 			'<column-name>' => true,
-			'<index-name>' => array('type' => '<type>', 'value' => '"<column-name>"'),
 			'<index-name>' => 'unique("<column-name>")'
+			'<index-name>' => array(
+				'type' => '<type>', 
+				'value' => '"<column-name>"'
+			),
 		);
 	}
 
+The `<column-name>` is used to put a standard non-unique index on the column specified. For complex or large tables 
+we recommend building the index to suite the requirements of your data.
+ 
 The `<index-name>` can be an arbitrary identifier in order to allow for more than one index on a specific database 
 column. The "advanced" notation supports more `<type>` notations. These vary between database drivers, but all of them 
 support the following:
 
- * `index`: Standard index
+ * `index`: Standard non unique index. 
  * `unique`: Index plus uniqueness constraint on the value
  * `fulltext`: Fulltext content index
 
-In order to use more database specific or complex index notations, we also support raw SQL as a value in the 
-`$indexes` definition. Keep in mind that using raw SQL is likely to make your code less portable between DBMSs.
-
 **mysite/code/MyTestObject.php**
 
 	:::php
@@ -50,6 +69,25 @@ In order to use more database specific or complex index notations, we also suppo
 		);
 	}
 
+## Complex/Composite Indexes
+For complex queries it may be necessary to define a complex or composite index on the supporting object. To create a 
+composite index, define the fields in the index order as a comma separated list. 
+
+*Note* Most DBMSs only use the leftmost prefix to optimise the query, try to ensure the order of the index and your 
+query parameters are the same. e.g.
+- index (col1) - `WHERE col1 = ?`
+- index (col1, col2) = `WHERE (col1 = ? AND col2 = ?)`
+- index (col1, col2, col3) = `WHERE (col1 = ? AND col2 = ? AND col3 = ?)`
+
+The index would not be used for a query `WHERE col2 = ?` or for `WHERE col1 = ? OR col2 = ?`
+
+As an alternative to a composite index, you can also create a hashed column which is a combination of information from 
+other columns. If this is indexed, smaller and reasonably unique it might be faster that an index on the whole column. 
+
+## Index Creation/Destruction
+Indexes are generated and removed automatically during a `dev/build`. Caution if you're working with large tables and 
+modify an index as the next `dev/build` will `DROP` the index, and then `ADD` it. 
+
 ## API Documentation
 
 * [api:DataObject]