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* Update docs/en/sql_reference/aggregate_functions/combinators.md
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74 lines
3.3 KiB
Markdown
74 lines
3.3 KiB
Markdown
# 表引擎 {#biao-yin-qing}
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表引擎（即表的类型）决定了：
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- 数据的存储方式和位置，写到哪里以及从哪里读取数据
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- 支持哪些查询以及如何支持。
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- 并发数据访问。
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- 索引的使用（如果存在）。
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- 是否可以执行多线程请求。
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- 数据复制参数。
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# 引擎类型 {#yin-qing-lei-xing}
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## MergeTree {#mergetree}
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适用于高负载任务的最通用和功能最强大的表引擎。这些引擎的共同特点是可以快速插入数据并进行后续的后台数据处理。 MergeTree系列引擎支持数据复制（使用[Replicated\*](mergetree-family/replication.md#table_engines-replication) 的引擎版本），分区和一些其他引擎不支持的其他功能。
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该类型的引擎：
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- [MergeTree](mergetree-family/mergetree.md#mergetree)
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- [ReplacingMergeTree](mergetree-family/replacingmergetree.md#replacingmergetree)
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- [SummingMergeTree](mergetree-family/summingmergetree.md#summingmergetree)
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- [AggregatingMergeTree](mergetree-family/aggregatingmergetree.md#aggregatingmergetree)
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- [CollapsingMergeTree](mergetree-family/collapsingmergetree.md#table_engine-collapsingmergetree)
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- [VersionedCollapsingMergeTree](mergetree-family/versionedcollapsingmergetree.md#versionedcollapsingmergetree)
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- [GraphiteMergeTree](mergetree-family/graphitemergetree.md#graphitemergetree)
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## 日志 {#log}
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具有最小功能的[轻量级引擎](log-family/index.md)。当您需要快速写入许多小表（最多约100万行）并在以后整体读取它们时，该类型的引擎是最有效的。
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该类型的引擎：
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- [TinyLog](log-family/tinylog.md#tinylog)
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- [StripeLog](log-family/stripelog.md#stripelog)
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- [Log](log-family/log.md#log)
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## 集成引擎 {#integration-engines}
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用于与其他的数据存储与处理系统集成的引擎。
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该类型的引擎：
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- [Kafka](integrations/kafka.md#kafka)
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- [MySQL](integrations/mysql.md#mysql)
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- [ODBC](integrations/odbc.md#table-engine-odbc)
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- [JDBC](integrations/jdbc.md#table-engine-jdbc)
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- [HDFS](integrations/hdfs.md#hdfs)
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## 用于其他特定功能的引擎 {#yong-yu-qi-ta-te-ding-gong-neng-de-yin-qing}
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该类型的引擎：
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- [Distributed](special/distributed.md#distributed)
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- [MaterializedView](special/materializedview.md#materializedview)
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- [Dictionary](special/dictionary.md#dictionary)
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- [Merge](special/merge.md#merge
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- [File](special/file.md#file)
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- [Null](special/null.md#null)
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- [Set](special/set.md#set)
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- [Join](special/join.md#join)
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- [URL](special/url.md#table_engines-url)
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- [View](special/view.md#table_engines-view)
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- [Memory](special/memory.md#memory)
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- [Buffer](special/buffer.md#buffer)
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# 虚拟列 {#table_engines-virtual_columns}
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虚拟列是表引擎组成的一部分，它在对应的表引擎的源代码中定义。
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您不能在 `CREATE TABLE` 中指定虚拟列，并且虚拟列不会包含在 `SHOW CREATE TABLE` 和 `DESCRIBE TABLE` 的查询结果中。虚拟列是只读的，所以您不能向虚拟列中写入数据。
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如果想要查询虚拟列中的数据，您必须在SELECT查询中包含虚拟列的名字。`SELECT *` 不会返回虚拟列的内容。
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若您创建的表中有一列与虚拟列的名字相同，那么虚拟列将不能再被访问。我们不建议您这样做。为了避免这种列名的冲突，虚拟列的名字一般都以下划线开头。
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