Add examples of useful dynamic templates to the docs. #17413

docs/reference/mapping/dynamic/templates.asciidoc

@@ -249,6 +249,148 @@ PUT my_index/my_type/1
 <1> The `english` field is mapped as a `string` field with the `english` analyzer.
 <2> The `count` field is mapped as a `long` field with `doc_values` disabled
 
+[[template-examples]]
+==== Template examples
+
+Here are some examples of potentially useful dynamic templates:
+
+===== Structured search
+
+By default elasticsearch will map string fields as a `text` field with a sub
+`keyword` field. However if you are only indexing structured content and not
+interested in full text search, you can make elasticsearch map your fields
+only as `keyword`s. Note that this means that in order to search those fields,
+you will have to search on the exact same value that was indexed.
+
+[source,js]
+--------------------------------------------------
+PUT my_index
+{
+  "mappings": {
+    "my_type": {
+      "dynamic_templates": [
+        {
+          "strings_as_keywords": {
+            "match_mapping_type": "string",
+            "mapping": {
+              "type": "keyword"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
+--------------------------------------------------
+
+===== `text`-only mappings for strings
+
+On the contrary to the previous example, if the only thing that you care about
+on your string fields is full-text search, and if you don't plan on running
+aggregations, sorting or exact search on your string fields, you could tell
+elasticsearch to map it only as a text field (which was the default behaviour
+before 5.0):
+
+[source,js]
+--------------------------------------------------
+PUT my_index
+{
+  "mappings": {
+    "my_type": {
+      "dynamic_templates": [
+        {
+          "strings_as_text": {
+            "match_mapping_type": "string",
+            "mapping": {
+              "type": "text"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
+--------------------------------------------------
+
+===== Disabled norms
+
+Norms are index-time scoring factors. If you do not care about scoring, which
+would be the case for instance if you never sort documents by score, you could
+disable the storage of these scoring factors in the index and save some space.
+
+[source,js]
+--------------------------------------------------
+PUT my_index
+{
+  "mappings": {
+    "my_type": {
+      "dynamic_templates": [
+        {
+          "strings_as_keywords": {
+            "match_mapping_type": "string",
+            "mapping": {
+              "type": "text",
+              "norms": false,
+              "fields": {
+                "keyword": {
+                  "type": "keyword",
+                  "ignore_above": 256
+                }
+              }
+            }
+          }
+        }
+      ]
+    }
+  }
+}
+--------------------------------------------------
+
+The sub `keyword` field appears in this template to be consistent with the
+default rules of dynamic mappings. Of course if you do not need them because 
+you don't need to perform exact search or aggregate on this field, you could
+remove it as described in the previous section.
+
+===== Time-series
+
+When doing time series analysis with elastisearch, it is common to have many
+numeric fields that you will often aggregate on but never filter on. In such a
+case, you could disable indexing on those fields to save disk space and also
+maybe gain some indexing speed:
+
+[source,js]
+--------------------------------------------------
+PUT my_index
+{
+  "mappings": {
+    "my_type": {
+      "dynamic_templates": [
+        {
+          "unindexed_longs": {
+            "match_mapping_type": "long",
+            "mapping": {
+              "type": "long",
+              "index": false
+            }
+          }
+        },
+        {
+          "unindexed_doubles": {
+            "match_mapping_type": "double",
+            "mapping": {
+              "type": "float", <1>
+              "index": false
+            }
+          }
+        }
+      ]
+    }
+  }
+}
+--------------------------------------------------
+<1> Like the default dynamic mapping rules, doubles are mapped as floats, which
+    are usually accurate enough, yet require half the disk space.
+
 
 [[override-default-template]]
 === Override default template