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

• _table — Contains the name of the table from which data was read. Type: String.

  You can set the constant conditions on _table in the WHERE/PREWHERE clause (for example, WHERE _table='xyz'). In this case the read operation is performed only for that tables where the condition on _table is satisfied, so the _table column acts as an index.

See Also

• Virtual columns

Original article