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64 lines
3.1 KiB
Markdown
64 lines
3.1 KiB
Markdown
# Merge
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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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The `Merge` engine accepts parameters: the database name and a regular expression for tables.
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Example:
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```sql
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Merge(hits, '^WatchLog')
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```
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Data will be read from the tables in the `hits` database that have names that match the regular expression '`^WatchLog`'.
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Instead of the database name, you can use a constant expression that returns a string. For example, `currentDatabase()`.
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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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When selecting tables to read, the `Merge` table itself will not be 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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Example 2:
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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.
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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 *
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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
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- `_table` — Contains the name of the table from which data was read. Type: [String](../../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](index.md#table_engines-virtual_columns)
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[Original article](https://clickhouse.yandex/docs/en/operations/table_engines/merge/) <!--hide-->
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