Added examples and extra info to projections

I have added two examples about how we can use projections

docs/en/sql-reference/statements/alter/projection.md

@@ -2,9 +2,134 @@
 slug: /en/sql-reference/statements/alter/projection
 sidebar_position: 49
 sidebar_label: PROJECTION
-title: "Manipulating Projections"
+title: "Projections"
 ---
 
+Projections store data in a format that optimizes query execution, this feature is useful if:
+You need to run queries on a column that is not a part of the primary key,
+For Pre-aggregate columns, it will reduce both computation and IO.
+
+You can define one or more projections for a table, and during the query analysis the projection with least data to scan will be selected by ClickHouse without modifying the query provided by the user.
+
+## Example filtering without using primary keys
+
+Creating the table:
+```
+CREATE TABLE visits_order
+(
+   `user_id` UInt64,
+   `user_name` String,
+   `pages_visited` Nullable(Float64),
+   `user_agent` String
+)
+ENGINE = MergeTree()
+PRIMARY KEY user_agent
+```
+Using `ALTER TABLE`, we could add the Projection to an existing table:
+```
+ALTER TABLE visits_order ADD PROJECTION user_name_projection (
+SELECT
+* 
+ORDER BY user_name
+)
+
+ALTER TABLE visits_order MATERIALIZE PROJECTION user_name_projection
+```
+Inserting the data:
+```
+INSERT INTO visits_order SELECT
+    number,
+    'test',
+    1.5 * (number / 2),
+    'Android'
+FROM numbers(1, 100);
+```
+
+The Projection will allow us to filter by `user_name` fast even if in the original Table `user_name` was not defined as a `PRIMARY_KEY`.
+At query time ClickHouse determined that less data will be processed if the projection is used, as the data is ordered by `user_name`.
+```
+SELECT
+    *
+FROM visits_order
+WHERE user_name='test'
+LIMIT 2
+```
+
+To verify that a query is using the projection, we could review the `system.query_log` table. On the `projections` field we have the name of the projection used or empty if none has been used:
+```
+SELECT query, projections FROM system.query_log WHERE query_id='<query_id>'
+```
+
+## Example for pre-aggregate query
+
+Creating the table with the Projection:
+```
+CREATE TABLE visits
+(
+   `user_id` UInt64,
+   `user_name` String,
+   `pages_visited` Nullable(Float64),
+   `user_agent` String,
+   PROJECTION projection_visits_by_user
+   (
+       SELECT
+           user_agent,
+           sum(pages_visited)
+       GROUP BY user_id, user_agent
+   )
+)
+ENGINE = MergeTree()
+ORDER BY user_agent
+```
+Inserting the data:
+```
+INSERT INTO visits SELECT
+    number,
+    'test',
+    1.5 * (number / 2),
+    'Android'
+FROM numbers(1, 100);
+```
+```
+INSERT INTO visits SELECT
+    number,
+    'test',
+    1. * (number / 2),
+   'IOS'
+FROM numbers(100, 500);
+```
+We will execute a first query using `GROUP BY` using the field `user_agent`, this query will not use the projection defined as the pre-aggregate do not match.
+```
+SELECT
+    user_agent,
+    count(DISTINCT user_id)
+FROM visits
+GROUP BY user_agent
+```
+
+To use the projection we could execute queries that select partially or all the pre-aggregate and `GROUP BY` fields.
+```
+SELECT
+    user_agent
+FROM visits
+WHERE user_id > 50 AND user_id < 150
+GROUP BY user_agent
+```
+```
+SELECT
+    user_agent,
+    sum(pages_visited)
+FROM visits
+GROUP BY user_id
+```
+
+As mentioned before, we could review the `system.query_log` table. On the `projections` field we have the name of the projection used or empty if none has been used:
+```
+SELECT query, projections FROM system.query_log WHERE query_id='<query_id>'
+```
+
+# Manipulating Projections
+
 The following operations with [projections](/docs/en/engines/table-engines/mergetree-family/mergetree.md/#projections) are available:
 
 ## ADD PROJECTION