ClickHouse/docs/en/operations/table_engines/merge.md

Merge

The Merge engine (not to be confused with MergeTree) does not store data itself, but allows reading from any number of other tables simultaneously. Reading is automatically parallelized. Writing to a table is not supported. When reading, the indexes of tables that are actually being read are used, if they exist. The Merge engine accepts parameters: the database name and a regular expression for tables.

Example:

Merge(hits, '^WatchLog')

Data will be read from the tables in the hits database that have names that match the regular expression '^WatchLog'.

Instead of the database name, you can use a constant expression that returns a string. For example, currentDatabase().

Regular expressions — re2 (supports a subset of PCRE), case-sensitive. See the notes about escaping symbols in regular expressions in the "match" section.

When selecting tables to read, the Merge table itself will not be selected, even if it matches the regex. This is to avoid loops. It is possible to create two Merge tables that will endlessly try to read each others' data, but this is not a good idea.

The typical way to use the Merge engine is for working with a large number of TinyLog tables as if with a single table.

Example 2:

Let's say you have a old table (WatchLog_old) and decided to change partitioning without moving data to a new table (WatchLog_new) and you need to see data from both tables.

CREATE TABLE WatchLog_old(date Date, UserId Int64, EventType String, Cnt UInt64) 
ENGINE=MergeTree(date, (UserId, EventType), 8192);
INSERT INTO WatchLog_old VALUES ('2018-01-01', 1, 'hit', 3);

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');

SELECT *
FROM WatchLog

┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
│ 2018-01-01 │      1 │ hit       │   3 │
└────────────┴────────┴───────────┴─────┘
┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
│ 2018-01-02 │      2 │ hit       │   3 │
└────────────┴────────┴───────────┴─────┘

Virtual Columns

Virtual columns are columns that are provided by the table engine, regardless of the table definition. In other words, these columns are not specified in CREATE TABLE, but they are accessible for SELECT.

Virtual columns differ from normal columns in the following ways:

• They are not specified in table definitions.
• Data can't be added to them with INSERT.
• When using INSERT without specifying the list of columns, virtual columns are ignored.
• They are not selected when using the asterisk (SELECT *).
• Virtual columns are not shown in SHOW CREATE TABLE and DESC TABLE queries.

The Merge type table contains a virtual _table column of the String type. (If the table already has a _table column, the virtual column is called _table1; if you already have _table1, it's called _table2, and so on.) It contains the name of the table that data was read from.

If the WHERE/PREWHERE clause contains conditions for the _table column that do not depend on other table columns (as one of the conjunction elements, or as an entire expression), these conditions are used as an index. The conditions are performed on a data set of table names to read data from, and the read operation will be performed from only those tables that the condition was triggered on.

Original article