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90 lines
3.7 KiB
Markdown
90 lines
3.7 KiB
Markdown
---
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slug: /en/engines/table-engines/special/merge
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sidebar_position: 30
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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
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CREATE TABLE ... Engine=Merge(db_name, tables_regexp)
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```
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## Engine Parameters
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### db_name
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`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()`,
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- `REGEXP(expression)`, where `expression` is a regular expression to match the DB names.
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### tables_regexp
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`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}
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When selecting tables to read, the `Merge` table itself is not selected, even if it matches the regex. This is to avoid loops.
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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}
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**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');
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```
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**Example 2**
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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);
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CREATE TABLE WatchLog as WatchLog_old ENGINE=Merge(currentDatabase(), '^WatchLog');
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SELECT * FROM WatchLog;
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```
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``` text
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┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
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│ 2018-01-01 │ 1 │ hit │ 3 │
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└────────────┴────────┴───────────┴─────┘
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┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
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│ 2018-01-02 │ 2 │ hit │ 3 │
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└────────────┴────────┴───────────┴─────┘
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```
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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).
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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.
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**See Also**
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- [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
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