Write to Merge storage

docs/en/engines/table-engines/special/merge.md

@@ -13,7 +13,7 @@ Reading is automatically parallelized. Writing to a table is not supported. When
 ## Creating a Table {#creating-a-table}
 
 ``` sql
-CREATE TABLE ... Engine=Merge(db_name, tables_regexp)
+CREATE TABLE ... Engine=Merge(db_name, tables_regexp [, table_to_write])
 ```
 
 ## Engine Parameters
@@ -32,6 +32,14 @@ CREATE TABLE ... Engine=Merge(db_name, tables_regexp)
 Regular expressions — [re2](https://github.com/google/re2) (supports a subset of PCRE), case-sensitive.
 See the notes about escaping symbols in regular expressions in the "match" section.
 
+### table_to_write {#table_to_write}
+
+`table_to_write` - Table name to write during inserts into `Merge` table.
+Possible values:
+    - `'db_name.table_name'` - insert into the specific table in the specific database.
+    - `'table_name'` - insert into table `db_name.table_name`. Allowed only when the first parameter `db_name` is not a regular expression.
+    - `auto` - insert into the last table passed to `tables_regexp` in lexicographical order. Allowed only when the first parameter `db_name` is not a regular expression.
+
 ## Usage {#usage}
 
 When selecting tables to read, the `Merge` table itself is not selected, even if it matches the regex. This is to avoid loops.
@@ -62,7 +70,7 @@ CREATE TABLE WatchLog_new(date Date, UserId Int64, EventType String, Cnt UInt64)
     ENGINE=MergeTree PARTITION BY date ORDER BY (UserId, EventType) SETTINGS index_granularity=8192;
 INSERT INTO WatchLog_new VALUES ('2018-01-02', 2, 'hit', 3);
 
-CREATE TABLE WatchLog as WatchLog_old ENGINE=Merge(currentDatabase(), '^WatchLog');
+CREATE TABLE WatchLog as WatchLog_old ENGINE=Merge(currentDatabase(), '^WatchLog', 'WatchLog_new');
 
 SELECT * FROM WatchLog;
 ```
@@ -76,6 +84,22 @@ SELECT * FROM WatchLog;
 └────────────┴────────┴───────────┴─────┘
 ```
 
+Insert to table `WatchLog` is going into table `WatchLog_new`
+```sql
+INSERT INTO WatchLog VALUES ('2018-01-03', 3, 'hit', 3);
+
+SELECT * FROM WatchLog_New;
+```
+
+```text
+┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
+│ 2018-01-02 │      2 │ hit       │   3 │
+└────────────┴────────┴───────────┴─────┘
+┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
+│ 2018-01-03 │      3 │ hit       │   3 │
+└────────────┴────────┴───────────┴─────┘
+```
+
 ## Virtual Columns {#virtual-columns}
 
 - `_table` — Contains the name of the table from which data was read. Type: [String](../../../sql-reference/data-types/string.md).