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spark-instrumented-optimizer/docs/sql-ref-syntax-aux-cache-cache-table.md
root1 b20754d9ee [SPARK-27545][SQL][DOC] Update the Documentation for CACHE TABLE and UNCACHE TABLE 
### 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>
2020-02-11 20:42:02 +08:00

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global CACHE TABLE CACHE TABLE Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

Description

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. This reduces scanning of the original files in future queries.

Syntax

{% highlight sql %} CACHE [ LAZY ] TABLE table_identifier [ OPTIONS ( 'storageLevel' [ = ] value ) ] [ [ AS ] query ] {% endhighlight %}

Parameters

LAZY
Only cache the table when it is first used, instead of immediately.
table_identifier
Specifies the table or view name to be cached. The table or view name may be optionally qualified with a database name.

Syntax: [ database_name. ] table_name
OPTIONS ( 'storageLevel' [ = ] value )
OPTIONS clause with storageLevel key and value pair. A Warning is issued when a key other than storageLevel is used. The valid options for storageLevel are:
  • NONE
  • DISK_ONLY
  • DISK_ONLY_2
  • MEMORY_ONLY
  • MEMORY_ONLY_2
  • MEMORY_ONLY_SER
  • MEMORY_ONLY_SER_2
  • MEMORY_AND_DISK
  • MEMORY_AND_DISK_2
  • MEMORY_AND_DISK_SER
  • MEMORY_AND_DISK_SER_2
  • OFF_HEAP
An Exception is thrown when an invalid value is set for storageLevel. If storageLevel is not explicitly set using OPTIONS clause, the default storageLevel is set to MEMORY_AND_DISK.
query
A query that produces the rows to be cached. It can be in one of following formats:
  • a SELECT statement
  • a TABLE statement
  • a FROM statement

Examples

{% highlight sql %} CACHE TABLE testCache OPTIONS ('storageLevel' 'DISK_ONLY') SELECT * FROM testData; {% endhighlight %}

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