2020-04-03 13:23:32 +00:00
---
2022-08-28 14:53:34 +00:00
slug: /en/engines/table-engines/special/merge
2022-04-09 13:29:05 +00:00
sidebar_position: 30
sidebar_label: Merge
2020-04-03 13:23:32 +00:00
---
2022-06-02 10:55:18 +00:00
# Merge Table Engine
2017-04-03 19:49:50 +00:00
2018-12-25 15:25:43 +00:00
The `Merge` engine (not to be confused with `MergeTree` ) does not store data itself, but allows reading from any number of other tables simultaneously.
2020-06-10 20:21:42 +00:00
2017-04-26 17:26:17 +00:00
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.
2020-06-10 20:21:42 +00:00
2021-11-01 10:39:25 +00:00
## Creating a Table {#creating-a-table}
2018-04-28 11:45:37 +00:00
2020-03-20 10:10:48 +00:00
``` sql
2022-04-09 13:29:05 +00:00
CREATE TABLE ... Engine=Merge(db_name, tables_regexp)
2017-12-28 15:13:23 +00:00
```
2017-06-13 04:15:47 +00:00
2022-06-24 16:16:20 +00:00
## Engine Parameters
2021-11-01 10:39:25 +00:00
2022-06-24 16:16:20 +00:00
### db_name
`db_name` — Possible values:
2022-06-02 10:55:18 +00:00
- database name,
2021-11-02 14:29:20 +00:00
- constant expression that returns a string with a database name, for example, `currentDatabase()` ,
- `REGEXP(expression)` , where `expression` is a regular expression to match the DB names.
2017-04-03 19:49:50 +00:00
2022-06-24 16:16:20 +00:00
### tables_regexp
`tables_regexp` — A regular expression to match the table names in the specified DB or DBs.
2017-04-03 19:49:50 +00:00
2018-04-23 06:20:21 +00:00
Regular expressions — [re2 ](https://github.com/google/re2 ) (supports a subset of PCRE), case-sensitive.
2021-11-01 10:39:25 +00:00
See the notes about escaping symbols in regular expressions in the "match" section.
2017-04-03 19:49:50 +00:00
2021-11-01 10:39:25 +00:00
## 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.
It is possible to create two `Merge` tables that will endlessly try to read each others' data, but this is not a good idea.
2017-04-03 19:49:50 +00:00
2018-12-25 15:25:43 +00:00
The typical way to use the `Merge` engine is for working with a large number of `TinyLog` tables as if with a single table.
2017-04-03 19:49:50 +00:00
2021-11-01 10:39:25 +00:00
## Examples {#examples}
**Example 1**
2018-10-04 20:58:28 +00:00
2021-11-02 09:01:01 +00:00
Consider two databases `ABC_corporate_site` and `ABC_store` . The `all_visitors` table will contain IDs from the tables `visitors` in both databases.
2018-10-04 20:58:28 +00:00
2020-03-20 10:10:48 +00:00
``` sql
2021-11-01 10:39:25 +00:00
CREATE TABLE all_visitors (id UInt32) ENGINE=Merge(REGEXP('ABC_*'), 'visitors');
```
**Example 2**
2021-11-02 09:01:01 +00:00
Let's say you have an 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.
2021-11-01 10:39:25 +00:00
``` sql
2022-06-02 10:55:18 +00:00
CREATE TABLE WatchLog_old(date Date, UserId Int64, EventType String, Cnt UInt64)
2021-11-01 10:39:25 +00:00
ENGINE=MergeTree(date, (UserId, EventType), 8192);
2018-10-03 17:43:47 +00:00
INSERT INTO WatchLog_old VALUES ('2018-01-01', 1, 'hit', 3);
2022-06-02 10:55:18 +00:00
CREATE TABLE WatchLog_new(date Date, UserId Int64, EventType String, Cnt UInt64)
2021-11-01 10:39:25 +00:00
ENGINE=MergeTree PARTITION BY date ORDER BY (UserId, EventType) SETTINGS index_granularity=8192;
2018-10-03 17:43:47 +00:00
INSERT INTO WatchLog_new VALUES ('2018-01-02', 2, 'hit', 3);
CREATE TABLE WatchLog as WatchLog_old ENGINE=Merge(currentDatabase(), '^WatchLog');
2021-11-01 10:39:25 +00:00
SELECT * FROM WatchLog;
2019-08-14 06:45:24 +00:00
```
2020-03-20 10:10:48 +00:00
``` text
2018-10-03 17:43:47 +00:00
┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
│ 2018-01-01 │ 1 │ hit │ 3 │
└────────────┴────────┴───────────┴─────┘
┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
│ 2018-01-02 │ 2 │ hit │ 3 │
└────────────┴────────┴───────────┴─────┘
```
2018-10-04 20:58:28 +00:00
2020-03-20 10:10:48 +00:00
## Virtual Columns {#virtual-columns}
2017-04-03 19:49:50 +00:00
2020-04-30 18:19:18 +00:00
- `_table` — Contains the name of the table from which data was read. Type: [String ](../../../sql-reference/data-types/string.md ).
2017-12-28 15:13:23 +00:00
2020-03-21 04:11:51 +00:00
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.
2019-05-20 12:23:07 +00:00
2019-08-14 06:45:24 +00:00
**See Also**
2018-09-04 11:18:59 +00:00
2020-06-18 08:24:31 +00:00
- [Virtual columns ](../../../engines/table-engines/special/index.md#table_engines-virtual_columns )
2021-11-01 10:39:25 +00:00
- [merge ](../../../sql-reference/table-functions/merge.md ) table function
2022-04-09 13:29:05 +00:00