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

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---
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slug: /en/engines/table-engines/special/merge
sidebar_position: 30
sidebar_label: Merge
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---
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# Merge Table Engine
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The `Merge` engine (not to be confused with `MergeTree`) does not store data itself, but allows reading from any number of other tables simultaneously.
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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.
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## Creating a Table {#creating-a-table}
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``` sql
CREATE TABLE ... Engine=Merge(db_name, tables_regexp)
```
CLICKHOUSE-2720: progress on website (#865) * update presentations * CLICKHOUSE-2936: redirect from clickhouse.yandex.ru and clickhouse.yandex.com * update submodule * lost files * CLICKHOUSE-2981: prefer sphinx docs over original reference * CLICKHOUSE-2981: docs styles more similar to main website + add flags to switch language links * update presentations * Less confusing directory structure (docs -> doc/reference/) * Minify sphinx docs too * Website release script: fail fast + pass docker hash on deploy * Do not underline links in docs * shorter * cleanup docker images * tune nginx config * CLICKHOUSE-3043: get rid of habrastorage links * Lost translation * CLICKHOUSE-2936: temporary client-side redirect * behaves weird in test * put redirect back * CLICKHOUSE-3047: copy docs txts to public too * move to proper file * remove old pages to avoid confusion * Remove reference redirect warning for now * Refresh README.md * Yellow buttons in docs * Use svg flags instead of unicode ones in docs * fix test website instance * Put flags to separate files * wrong flag * Copy Yandex.Metrica introduction from main page to docs * Yet another home page structure change, couple new blocks (CLICKHOUSE-3045) * Update Contacts section * CLICKHOUSE-2849: more detailed legal information * CLICKHOUSE-2978 preparation - split by files * More changes in Contacts block * Tune texts on index page * update presentations * One more benchmark * Add usage sections to index page, adapted from slides * Get the roadmap started, based on slides from last ClickHouse Meetup * CLICKHOUSE-2977: some rendering tuning * Get rid of excessive section in the end of getting started * Make headers linkable * CLICKHOUSE-2981: links to editing reference - https://github.com/yandex/ClickHouse/issues/849 * CLICKHOUSE-2981: fix mobile styles in docs * Ban crawling of duplicating docs * Open some external links in new tab * Ban old docs too * Lots of trivial fixes in english docs * Lots of trivial fixes in russian docs * Remove getting started copies in markdown * Add Yandex.Webmaster * Fix some sphinx warnings * More warnings fixed in english docs * More sphinx warnings fixed * Add code-block:: text * More code-block:: text * These headers look not that well * Better switch between documentation languages * merge use_case.rst into ya_metrika_task.rst * Edit the agg_functions.rst texts * Add lost empty lines
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## Engine Parameters
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### db_name
`db_name` — Possible values:
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- database name,
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- 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.
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### tables_regexp
`tables_regexp` — A regular expression to match the table names in the specified DB or DBs.
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Regular expressions — [re2](https://github.com/google/re2) (supports a subset of PCRE), case-sensitive.
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See the notes about escaping symbols in regular expressions in the "match" section.
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## 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.
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The typical way to use the `Merge` engine is for working with a large number of `TinyLog` tables as if with a single table.
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## Examples {#examples}
**Example 1**
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Consider two databases `ABC_corporate_site` and `ABC_store`. The `all_visitors` table will contain IDs from the tables `visitors` in both databases.
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``` sql
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CREATE TABLE all_visitors (id UInt32) ENGINE=Merge(REGEXP('ABC_*'), 'visitors');
```
**Example 2**
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.
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``` sql
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CREATE TABLE WatchLog_old(date Date, UserId Int64, EventType String, Cnt UInt64)
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ENGINE=MergeTree(date, (UserId, EventType), 8192);
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INSERT INTO WatchLog_old VALUES ('2018-01-01', 1, 'hit', 3);
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CREATE TABLE WatchLog_new(date Date, UserId Int64, EventType String, Cnt UInt64)
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ENGINE=MergeTree PARTITION BY date ORDER BY (UserId, EventType) SETTINGS index_granularity=8192;
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INSERT INTO WatchLog_new VALUES ('2018-01-02', 2, 'hit', 3);
CREATE TABLE WatchLog as WatchLog_old ENGINE=Merge(currentDatabase(), '^WatchLog');
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SELECT * FROM WatchLog;
```
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``` text
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┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
│ 2018-01-01 │ 1 │ hit │ 3 │
└────────────┴────────┴───────────┴─────┘
┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
│ 2018-01-02 │ 2 │ hit │ 3 │
└────────────┴────────┴───────────┴─────┘
```
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## Virtual Columns {#virtual-columns}
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- `_table` — Contains the name of the table from which data was read. Type: [String](../../../sql-reference/data-types/string.md).
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](../../../engines/table-engines/special/index.md#table_engines-virtual_columns)
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- [merge](../../../sql-reference/table-functions/merge.md) table function