b20754d9ee
### What changes were proposed in this pull request? Document updated for `CACHE TABLE` & `UNCACHE TABLE` ### Why are the changes needed? Cache table creates a temp view while caching data using `CACHE TABLE name AS query`. `UNCACHE TABLE` does not remove this temp view. These things were not mentioned in the existing doc for `CACHE TABLE` & `UNCACHE TABLE`. ### Does this PR introduce any user-facing change? Document updated for `CACHE TABLE` & `UNCACHE TABLE` command. ### How was this patch tested? Manually Closes #27090 from iRakson/SPARK-27545. Lead-authored-by: root1 <raksonrakesh@gmail.com> Co-authored-by: iRakson <raksonrakesh@gmail.com> Signed-off-by: Wenchen Fan <wenchen@databricks.com>
92 lines
3.4 KiB
Markdown
92 lines
3.4 KiB
Markdown
---
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layout: global
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title: CACHE TABLE
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displayTitle: CACHE TABLE
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license: |
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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---
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### Description
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`CACHE TABLE` statement caches contents of a table or output of a query with the given storage level. If a query is cached, then a temp view will be created for this query.
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This reduces scanning of the original files in future queries.
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### Syntax
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{% highlight sql %}
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CACHE [ LAZY ] TABLE table_identifier
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[ OPTIONS ( 'storageLevel' [ = ] value ) ] [ [ AS ] query ]
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{% endhighlight %}
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### Parameters
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<dl>
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<dt><code><em>LAZY</em></code></dt>
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<dd>Only cache the table when it is first used, instead of immediately.</dd>
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</dl>
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<dl>
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<dt><code><em>table_identifier</em></code></dt>
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<dd>
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Specifies the table or view name to be cached. The table or view name may be optionally qualified with a database name.<br><br>
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<b>Syntax:</b>
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<code>
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[ database_name. ] table_name
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</code>
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</dd>
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</dl>
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<dl>
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<dt><code><em>OPTIONS ( 'storageLevel' [ = ] value )</em></code></dt>
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<dd>
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<code>OPTIONS</code> clause with <code>storageLevel</code> key and value pair. A Warning is issued when a key other than <code>storageLevel</code> is used. The valid options for <code>storageLevel</code> are:
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<ul>
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<li><code>NONE</code></li>
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<li><code>DISK_ONLY</code></li>
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<li><code>DISK_ONLY_2</code></li>
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<li><code>MEMORY_ONLY</code></li>
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<li><code>MEMORY_ONLY_2</code></li>
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<li><code>MEMORY_ONLY_SER</code></li>
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<li><code>MEMORY_ONLY_SER_2</code></li>
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<li><code>MEMORY_AND_DISK</code></li>
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<li><code>MEMORY_AND_DISK_2</code></li>
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<li><code>MEMORY_AND_DISK_SER</code></li>
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<li><code>MEMORY_AND_DISK_SER_2</code></li>
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<li><code>OFF_HEAP</code></li>
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</ul>
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An Exception is thrown when an invalid value is set for <code>storageLevel</code>. If <code>storageLevel</code> is not explicitly set using <code>OPTIONS</code> clause, the default <code>storageLevel</code> is set to <code>MEMORY_AND_DISK</code>.
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</dd>
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</dl>
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<dl>
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<dt><code><em>query</em></code></dt>
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<dd>A query that produces the rows to be cached. It can be in one of following formats:
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<ul>
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<li>a <code>SELECT</code> statement</li>
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<li>a <code>TABLE</code> statement</li>
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<li>a <code>FROM</code> statement</li>
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</ul>
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</dd>
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</dl>
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### Examples
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{% highlight sql %}
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CACHE TABLE testCache OPTIONS ('storageLevel' 'DISK_ONLY') SELECT * FROM testData;
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{% endhighlight %}
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### Related Statements
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* [CLEAR CACHE](sql-ref-syntax-aux-cache-clear-cache.html)
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* [UNCACHE TABLE](sql-ref-syntax-aux-cache-uncache-table.html)
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* [REFRESH TABLE](sql-ref-syntax-aux-refresh-table.html)
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