docs/topics/impala_views.xml - impala - Git at Google

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 <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
 <concept rev="1.1" id="views">

   <title>Overview of Impala Views</title>
   <titlealts audience="PDF"><navtitle>Views</navtitle></titlealts>
   <prolog>
     <metadata>
       <data name="Category" value="Impala"/>
       <data name="Category" value="SQL"/>
       <data name="Category" value="Data Analysts"/>
       <data name="Category" value="Developers"/>
       <data name="Category" value="Querying"/>
       <data name="Category" value="Tables"/>
       <data name="Category" value="Schemas"/>
       <data name="Category" value="Views"/>
     </metadata>
   </prolog>

   <conbody>

     <p>
       Views are lightweight logical constructs that act as aliases for queries. You can specify a view name in a
       query (a <codeph>SELECT</codeph> statement or the <codeph>SELECT</codeph> portion of an
       <codeph>INSERT</codeph> statement) where you would usually specify a table name.
     </p>

     <p>
       A view lets you:
     </p>

     <ul>
       <li>
         Issue complicated queries with compact and simple syntax:
 <codeblock>-- Take a complicated reporting query, plug it into a CREATE VIEW statement...
 create view v1 as select c1, c2, avg(c3) from t1 group by c3 order by c1 desc limit 10;
 -- ... and now you can produce the report with 1 line of code.
 select * from v1;</codeblock>
       </li>

       <li>
         Reduce maintenance, by avoiding the duplication of complicated queries across multiple applications in
         multiple languages:
 <codeblock>create view v2 as select t1.c1, t1.c2, t2.c3 from t1 join t2 on (t1.id = t2.id);
 -- This simple query is safer to embed in reporting applications than the longer query above.
 -- The view definition can remain stable even if the structure of the underlying tables changes.
 select c1, c2, c3 from v2;</codeblock>
       </li>

       <li>
         Build a new, more refined query on top of the original query by adding new clauses, select-list
         expressions, function calls, and so on:
 <codeblock>create view average_price_by_category as select category, avg(price) as avg_price from products group by category;
 create view expensive_categories as select category, avg_price from average_price_by_category order by avg_price desc limit 10000;
 create view top_10_expensive_categories as select category, avg_price from expensive_categories limit 10;</codeblock>
         This technique lets you build up several more or less granular variations of the same query, and switch
         between them when appropriate.
 <!-- My original assumption was confirmed correct by Alex: outer ORDER BY not actually needed.
 In this case, we put an <codeph>ORDER BY</codeph> clause on the <q>top 10</q> view, even though there was already an <codeph>ORDER BY</codeph>
 on the <q>top 10000</q> view, because when a query is executed in parallel and distributed among multiple nodes, the ordering is only
 guaranteed if there is an <codeph>ORDER BY</codeph> clause at the outermost level.
 -->
       </li>

       <li>
         Set up aliases with intuitive names for tables, columns, result sets from joins, and so on:
 <codeblock>-- The original tables might have cryptic names inherited from a legacy system.
 create view action_items as select rrptsk as assignee, treq as due_date, dmisc as notes from vxy_t1_br;
 -- You can leave original names for compatibility, build new applications using more intuitive ones.
 select assignee, due_date, notes from action_items;</codeblock>
       </li>

       <li>
         Swap tables with others that use different file formats, partitioning schemes, and so on without any
         downtime for data copying or conversion:
 <codeblock>create table slow (x int, s string) stored as textfile;
 create view report as select s from slow where x between 20 and 30;
 -- Query is kind of slow due to inefficient table definition, but it works.
 select * from report;

 create table fast (s string) partitioned by (x int) stored as parquet;
 -- ...Copy data from SLOW to FAST. Queries against REPORT view continue to work...

 -- After changing the view definition, queries will be faster due to partitioning,
 -- binary format, and compression in the new table.
 alter view report as select s from fast where x between 20 and 30;
 select * from report;</codeblock>
       </li>

       <li>
         Avoid coding lengthy subqueries and repeating the same subquery text in many other queries.
       </li>

       <li rev="2.3.0 collevelauth">
         Set up fine-grained security where a user can query some columns from a table but not other columns.
         Because <keyword keyref="impala23_full"/> and higher support column-level authorization, this technique is no longer
         required. If you formerly implemented column-level security through views, see
         <xref keyref="sg_hive_sql"/> for details about the column-level authorization feature.
       </li>
     </ul>

     <p>
       The SQL statements that configure views are <xref href="impala_create_view.xml#create_view"/>,
       <xref href="impala_alter_view.xml#alter_view"/>, and <xref href="impala_drop_view.xml#drop_view"/>. You can
       specify view names when querying data (<xref href="impala_select.xml#select"/>) and copying data from one
       table to another (<xref href="impala_insert.xml#insert"/>). The <xref href="impala_with.xml#with">WITH</xref>
       clause creates an inline view, that only exists for the duration of a single query.
     </p>

 <codeblock>[localhost:21000] &gt; create view trivial as select * from customer;
 [localhost:21000] &gt; create view some_columns as select c_first_name, c_last_name, c_login from customer;
 [localhost:21000] &gt; select * from some_columns limit 5;
 Query finished, fetching results ...
 +--------------+-------------+---------+
 | c_first_name | c_last_name | c_login |
 +--------------+-------------+---------+
 | Javier       | Lewis       |         |
 | Amy          | Moses       |         |
 | Latisha      | Hamilton    |         |
 | Michael      | White       |         |
 | Robert       | Moran       |         |
 +--------------+-------------+---------+
 [localhost:21000] &gt; create view ordered_results as select * from some_columns order by c_last_name desc, c_first_name desc limit 1000;
 [localhost:21000] &gt; select * from ordered_results limit 5;
 Query: select * from ordered_results limit 5
 Query finished, fetching results ...
 +--------------+-------------+---------+
 | c_first_name | c_last_name | c_login |
 +--------------+-------------+---------+
 | Thomas       | Zuniga      |         |
 | Sarah        | Zuniga      |         |
 | Norma        | Zuniga      |         |
 | Lloyd        | Zuniga      |         |
 | Lisa         | Zuniga      |         |
 +--------------+-------------+---------+
 Returned 5 row(s) in 0.48s</codeblock>

     <p>
       The previous example uses descending order for <codeph>ORDERED_RESULTS</codeph> because in the sample TPCD-H
       data, there are some rows with empty strings for both <codeph>C_FIRST_NAME</codeph> and
       <codeph>C_LAST_NAME</codeph>, making the lowest-ordered names unuseful in a sample query.
     </p>

 <codeblock>create view visitors_by_day as select day, count(distinct visitors) as howmany from web_traffic group by day;
 create view top_10_days as select day, howmany from visitors_by_day order by howmany limit 10;
 select * from top_10_days;</codeblock>

     <p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>

     <p conref="../shared/impala_common.xml#common/describe_formatted_view"/>

     <p conref="../shared/impala_common.xml#common/create_table_like_view"/>

     <p conref="../shared/impala_common.xml#common/complex_types_blurb"/>

     <p conref="../shared/impala_common.xml#common/complex_types_views"/>

     <p conref="../shared/impala_common.xml#common/straight_join_nested_queries"/>

     <p conref="../shared/impala_common.xml#common/restrictions_blurb"/>

     <ul>
       <li>
         <p>
           You cannot insert into an Impala view. (In some database systems, this operation is allowed and inserts
           rows into the base table.) You can use a view name on the right-hand side of an <codeph>INSERT</codeph>
           statement, in the <codeph>SELECT</codeph> part.
         </p>
       </li>

       <li>
 <!-- This same text is conref'ed in the #views and the #partition_pruning topics. -->
         <p conref="../shared/impala_common.xml#common/partitions_and_views"/>
       </li>

       <li rev="1.4.0">
         <p conref="../shared/impala_common.xml#common/order_by_view_restriction"/>
       </li>
       <li>
         <p conref="../shared/impala_common.xml#common/tablesample_caveat"/>
       </li>
     </ul>

     <p>
       <b>Related statements:</b> <xref href="impala_create_view.xml#create_view"/>,
       <xref href="impala_alter_view.xml#alter_view"/>, <xref href="impala_drop_view.xml#drop_view"/>
     </p>
   </conbody>
 </concept>
	<?xml version="1.0" encoding="UTF-8"?>
	<!--
	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.
	-->
	<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
	<concept rev="1.1" id="views">

	<title>Overview of Impala Views</title>
	<titlealts audience="PDF"><navtitle>Views</navtitle></titlealts>
	<prolog>
	<metadata>
	<data name="Category" value="Impala"/>
	<data name="Category" value="SQL"/>
	<data name="Category" value="Data Analysts"/>
	<data name="Category" value="Developers"/>
	<data name="Category" value="Querying"/>
	<data name="Category" value="Tables"/>
	<data name="Category" value="Schemas"/>
	<data name="Category" value="Views"/>
	</metadata>
	</prolog>

	<conbody>

	<p>
	Views are lightweight logical constructs that act as aliases for queries. You can specify a view name in a
	query (a <codeph>SELECT</codeph> statement or the <codeph>SELECT</codeph> portion of an
	<codeph>INSERT</codeph> statement) where you would usually specify a table name.
	</p>

	<p>
	A view lets you:
	</p>

	<ul>
	<li>
	Issue complicated queries with compact and simple syntax:
	<codeblock>-- Take a complicated reporting query, plug it into a CREATE VIEW statement...
	create view v1 as select c1, c2, avg(c3) from t1 group by c3 order by c1 desc limit 10;
	-- ... and now you can produce the report with 1 line of code.
	select * from v1;</codeblock>
	</li>

	<li>
	Reduce maintenance, by avoiding the duplication of complicated queries across multiple applications in
	multiple languages:
	<codeblock>create view v2 as select t1.c1, t1.c2, t2.c3 from t1 join t2 on (t1.id = t2.id);
	-- This simple query is safer to embed in reporting applications than the longer query above.
	-- The view definition can remain stable even if the structure of the underlying tables changes.
	select c1, c2, c3 from v2;</codeblock>
	</li>

	<li>
	Build a new, more refined query on top of the original query by adding new clauses, select-list
	expressions, function calls, and so on:
	<codeblock>create view average_price_by_category as select category, avg(price) as avg_price from products group by category;
	create view expensive_categories as select category, avg_price from average_price_by_category order by avg_price desc limit 10000;
	create view top_10_expensive_categories as select category, avg_price from expensive_categories limit 10;</codeblock>
	This technique lets you build up several more or less granular variations of the same query, and switch
	between them when appropriate.
	<!-- My original assumption was confirmed correct by Alex: outer ORDER BY not actually needed.
	In this case, we put an <codeph>ORDER BY</codeph> clause on the <q>top 10</q> view, even though there was already an <codeph>ORDER BY</codeph>
	on the <q>top 10000</q> view, because when a query is executed in parallel and distributed among multiple nodes, the ordering is only
	guaranteed if there is an <codeph>ORDER BY</codeph> clause at the outermost level.
	-->
	</li>

	<li>
	Set up aliases with intuitive names for tables, columns, result sets from joins, and so on:
	<codeblock>-- The original tables might have cryptic names inherited from a legacy system.
	create view action_items as select rrptsk as assignee, treq as due_date, dmisc as notes from vxy_t1_br;
	-- You can leave original names for compatibility, build new applications using more intuitive ones.
	select assignee, due_date, notes from action_items;</codeblock>
	</li>

	<li>
	Swap tables with others that use different file formats, partitioning schemes, and so on without any
	downtime for data copying or conversion:
	<codeblock>create table slow (x int, s string) stored as textfile;
	create view report as select s from slow where x between 20 and 30;
	-- Query is kind of slow due to inefficient table definition, but it works.
	select * from report;

	create table fast (s string) partitioned by (x int) stored as parquet;
	-- ...Copy data from SLOW to FAST. Queries against REPORT view continue to work...

	-- After changing the view definition, queries will be faster due to partitioning,
	-- binary format, and compression in the new table.
	alter view report as select s from fast where x between 20 and 30;
	select * from report;</codeblock>
	</li>

	<li>
	Avoid coding lengthy subqueries and repeating the same subquery text in many other queries.
	</li>

	<li rev="2.3.0 collevelauth">
	Set up fine-grained security where a user can query some columns from a table but not other columns.
	Because <keyword keyref="impala23_full"/> and higher support column-level authorization, this technique is no longer
	required. If you formerly implemented column-level security through views, see
	<xref keyref="sg_hive_sql"/> for details about the column-level authorization feature.
	</li>
	</ul>

	<p>
	The SQL statements that configure views are <xref href="impala_create_view.xml#create_view"/>,
	<xref href="impala_alter_view.xml#alter_view"/>, and <xref href="impala_drop_view.xml#drop_view"/>. You can
	specify view names when querying data (<xref href="impala_select.xml#select"/>) and copying data from one
	table to another (<xref href="impala_insert.xml#insert"/>). The <xref href="impala_with.xml#with">WITH</xref>
	clause creates an inline view, that only exists for the duration of a single query.
	</p>

	<codeblock>[localhost:21000] > create view trivial as select * from customer;
	[localhost:21000] > create view some_columns as select c_first_name, c_last_name, c_login from customer;
	[localhost:21000] > select * from some_columns limit 5;
	Query finished, fetching results ...
	+--------------+-------------+---------+
	\| c_first_name \| c_last_name \| c_login \|
	+--------------+-------------+---------+
	\| Javier \| Lewis \| \|
	\| Amy \| Moses \| \|
	\| Latisha \| Hamilton \| \|
	\| Michael \| White \| \|
	\| Robert \| Moran \| \|
	+--------------+-------------+---------+
	[localhost:21000] > create view ordered_results as select * from some_columns order by c_last_name desc, c_first_name desc limit 1000;
	[localhost:21000] > select * from ordered_results limit 5;
	Query: select * from ordered_results limit 5
	Query finished, fetching results ...
	+--------------+-------------+---------+
	\| c_first_name \| c_last_name \| c_login \|
	+--------------+-------------+---------+
	\| Thomas \| Zuniga \| \|
	\| Sarah \| Zuniga \| \|
	\| Norma \| Zuniga \| \|
	\| Lloyd \| Zuniga \| \|
	\| Lisa \| Zuniga \| \|
	+--------------+-------------+---------+
	Returned 5 row(s) in 0.48s</codeblock>

	<p>
	The previous example uses descending order for <codeph>ORDERED_RESULTS</codeph> because in the sample TPCD-H
	data, there are some rows with empty strings for both <codeph>C_FIRST_NAME</codeph> and
	<codeph>C_LAST_NAME</codeph>, making the lowest-ordered names unuseful in a sample query.
	</p>

	<codeblock>create view visitors_by_day as select day, count(distinct visitors) as howmany from web_traffic group by day;
	create view top_10_days as select day, howmany from visitors_by_day order by howmany limit 10;
	select * from top_10_days;</codeblock>

	<p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>

	<p conref="../shared/impala_common.xml#common/describe_formatted_view"/>

	<p conref="../shared/impala_common.xml#common/create_table_like_view"/>

	<p conref="../shared/impala_common.xml#common/complex_types_blurb"/>

	<p conref="../shared/impala_common.xml#common/complex_types_views"/>

	<p conref="../shared/impala_common.xml#common/straight_join_nested_queries"/>

	<p conref="../shared/impala_common.xml#common/restrictions_blurb"/>

	<ul>
	<li>
	<p>
	You cannot insert into an Impala view. (In some database systems, this operation is allowed and inserts
	rows into the base table.) You can use a view name on the right-hand side of an <codeph>INSERT</codeph>
	statement, in the <codeph>SELECT</codeph> part.
	</p>
	</li>

	<li>
	<!-- This same text is conref'ed in the #views and the #partition_pruning topics. -->
	<p conref="../shared/impala_common.xml#common/partitions_and_views"/>
	</li>

	<li rev="1.4.0">
	<p conref="../shared/impala_common.xml#common/order_by_view_restriction"/>
	</li>
	<li>
	<p conref="../shared/impala_common.xml#common/tablesample_caveat"/>
	</li>
	</ul>

	<p>
	<b>Related statements:</b> <xref href="impala_create_view.xml#create_view"/>,
	<xref href="impala_alter_view.xml#alter_view"/>, <xref href="impala_drop_view.xml#drop_view"/>
	</p>
	</conbody>
	</concept>