impala/docs/topics/impala_order_by.xml

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

  <title>ORDER BY Clause</title>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      The familiar <codeph>ORDER BY</codeph> clause of a <codeph>SELECT</codeph> statement sorts the result set
      based on the values from one or more columns.
    </p>

    <p>
      For distributed queries, this is a relatively expensive operation, because the entire result set must be
      produced and transferred to one node before the sorting can happen. This can require more memory capacity
      than a query without <codeph>ORDER BY</codeph>. Even if the query takes approximately the same time to finish
      with or without the <codeph>ORDER BY</codeph> clause, subjectively it can appear slower because no results
      are available until all processing is finished, rather than results coming back gradually as rows matching
      the <codeph>WHERE</codeph> clause are found. Therefore, if you only need the first N results from the sorted
      result set, also include the <codeph>LIMIT</codeph> clause, which reduces network overhead and the memory
      requirement on the coordinator node.
    </p>

    <note>
      <p rev="1.4.0 obwl">
        In Impala 1.4.0 and higher, the <codeph>LIMIT</codeph> clause is now optional (rather than required) for
        queries that use the <codeph>ORDER BY</codeph> clause. Impala automatically uses a temporary disk work area
        to perform the sort if the sort operation would otherwise exceed the Impala memory limit for a particular
        DataNode.
      </p>
    </note>

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

    <p>
      The full syntax for the <codeph>ORDER BY</codeph> clause is:
    </p>

<codeblock rev="1.2.1">ORDER BY <varname>col_ref</varname> [, <varname>col_ref</varname> ...] [ASC | DESC] [NULLS FIRST | NULLS LAST]

col_ref ::= <varname>column_name</varname> | <varname>integer_literal</varname>
</codeblock>

    <p>
      Although the most common usage is <codeph>ORDER BY <varname>column_name</varname></codeph>, you can also
      specify <codeph>ORDER BY 1</codeph> to sort by the first column of the result set, <codeph>ORDER BY
      2</codeph> to sort by the second column, and so on. The number must be a numeric literal, not some other kind
      of constant expression. (If the argument is some other expression, even a <codeph>STRING</codeph> value, the
      query succeeds but the order of results is undefined.)
    </p>

    <p>
      <codeph>ORDER BY <varname>column_number</varname></codeph> can only be used when the query explicitly lists
      the columns in the <codeph>SELECT</codeph> list, not with <codeph>SELECT *</codeph> queries.
    </p>

    <p>
      <b>Ascending and descending sorts:</b>
    </p>

    <p>
      The default sort order (the same as using the <codeph>ASC</codeph> keyword) puts the smallest values at the
      start of the result set, and the largest values at the end. Specifying the <codeph>DESC</codeph> keyword
      reverses that order.
    </p>

    <p>
      <b>Sort order for NULL values:</b>
    </p>

    <p rev="1.2.1">
      See <xref href="impala_literals.xml#null"/> for details about how <codeph>NULL</codeph> values are positioned
      in the sorted result set, and how to use the <codeph>NULLS FIRST</codeph> and <codeph>NULLS LAST</codeph>
      clauses. (The sort position for <codeph>NULL</codeph> values in <codeph>ORDER BY ... DESC</codeph> queries is
      changed in Impala 1.2.1 and higher to be more standards-compliant, and the <codeph>NULLS FIRST</codeph> and
      <codeph>NULLS LAST</codeph> keywords are new in Impala 1.2.1.)
    </p>

    <p rev="obwl" conref="../shared/impala_common.xml#common/order_by_limit"/>

    <!-- Good to show an example of cases where ORDER BY does and doesn't work with complex types. -->
    <p conref="../shared/impala_common.xml#common/complex_types_blurb"/>

    <p rev="2.3.0">
      In <keyword keyref="impala23_full"/> and higher, the complex data types <codeph>STRUCT</codeph>,
      <codeph>ARRAY</codeph>, and <codeph>MAP</codeph> are available. These columns cannot
      be referenced directly in the <codeph>ORDER BY</codeph> clause.
      When you query a complex type column, you use join notation to <q>unpack</q> the elements
      of the complex type, and within the join query you can include an <codeph>ORDER BY</codeph>
      clause to control the order in the result set of the scalar elements from the complex type.
      See <xref href="impala_complex_types.xml#complex_types"/> for details about Impala support for complex types.
    </p>

    <p>
      The following query shows how a complex type column cannot be directly used in an <codeph>ORDER BY</codeph> clause:
    </p>

<codeblock>CREATE TABLE games (id BIGINT, score ARRAY &lt;BIGINT&gt;) STORED AS PARQUET;
...use LOAD DATA to load externally created Parquet files into the table...
SELECT id FROM games ORDER BY score DESC;
ERROR: AnalysisException: ORDER BY expression 'score' with complex type 'ARRAY&lt;BIGINT&gt;' is not supported.
</codeblock>

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

    <p>
      The following query retrieves the user ID and score, only for scores greater than one million,
      with the highest scores for each user listed first.
      Because the individual array elements are now represented as separate rows in the result set,
      they can be used in the <codeph>ORDER BY</codeph> clause, referenced using the <codeph>ITEM</codeph>
      pseudocolumn that represents each array element.
    </p>

<codeblock>SELECT id, item FROM games, games.score
  WHERE item &gt; 1000000
ORDER BY id, item desc;
</codeblock>

    <p>
      The following queries use similar <codeph>ORDER BY</codeph> techniques with variations of the <codeph>GAMES</codeph>
      table, where the complex type is an <codeph>ARRAY</codeph> containing <codeph>STRUCT</codeph> or <codeph>MAP</codeph>
      elements to represent additional details about each game that was played.
      For an array of structures, the fields of the structure are referenced as <codeph>ITEM.<varname>field_name</varname></codeph>.
      For an array of maps, the keys and values within each array element are referenced as <codeph>ITEM.KEY</codeph>
      and <codeph>ITEM.VALUE</codeph>.
    </p>

<codeblock>CREATE TABLE games2 (id BIGINT, play array &lt; struct &lt;game_name: string, score: BIGINT, high_score: boolean&gt; &gt;) STORED AS PARQUET
...use LOAD DATA to load externally created Parquet files into the table...
SELECT id, item.game_name, item.score FROM games2, games2.play
  WHERE item.score &gt; 1000000
ORDER BY id, item.score DESC;

CREATE TABLE games3 (id BIGINT, play ARRAY &lt; MAP &lt;STRING, BIGINT&gt; &gt;) STORED AS PARQUET;
...use LOAD DATA to load externally created Parquet files into the table...
SELECT id, info.key AS k, info.value AS v from games3, games3.play AS plays, games3.play.item AS info
  WHERE info.KEY = 'score' AND info.VALUE &gt; 1000000
ORDER BY id, info.value desc;
</codeblock>

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

    <p>
      Although the <codeph>LIMIT</codeph> clause is now optional on <codeph>ORDER BY</codeph> queries, if your
      query only needs some number of rows that you can predict in advance, use the <codeph>LIMIT</codeph> clause
      to reduce unnecessary processing. For example, if the query has a clause <codeph>LIMIT 10</codeph>, each data
      node sorts its portion of the relevant result set and only returns 10 rows to the coordinator node. The
      coordinator node picks the 10 highest or lowest row values out of this small intermediate result set.
    </p>

    <p>
      If an <codeph>ORDER BY</codeph> clause is applied to an early phase of query processing, such as a subquery
      or a view definition, Impala ignores the <codeph>ORDER BY</codeph> clause. To get ordered results from a
      subquery or view, apply an <codeph>ORDER BY</codeph> clause to the outermost or final <codeph>SELECT</codeph>
      level.
    </p>

    <p>
      <codeph>ORDER BY</codeph> is often used in combination with <codeph>LIMIT</codeph> to perform <q>top-N</q>
      queries:
    </p>

<codeblock>SELECT user_id AS "Top 10 Visitors", SUM(page_views) FROM web_stats
  GROUP BY page_views, user_id
  ORDER BY SUM(page_views) DESC LIMIT 10;
</codeblock>

    <p>
      <codeph>ORDER BY</codeph> is sometimes used in combination with <codeph>OFFSET</codeph> and
      <codeph>LIMIT</codeph> to paginate query results, although it is relatively inefficient to issue multiple
      queries like this against the large tables typically used with Impala:
    </p>

<codeblock>SELECT page_title AS "Page 1 of search results", page_url FROM search_content
  WHERE LOWER(page_title) LIKE '%game%')
  ORDER BY page_title LIMIT 10 OFFSET 0;
SELECT page_title AS "Page 2 of search results", page_url FROM search_content
  WHERE LOWER(page_title) LIKE '%game%')
  ORDER BY page_title LIMIT 10 OFFSET 10;
SELECT page_title AS "Page 3 of search results", page_url FROM search_content
  WHERE LOWER(page_title) LIKE '%game%')
  ORDER BY page_title LIMIT 10 OFFSET 20;
</codeblock>

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

    <p>
      Impala sorts the intermediate results of an <codeph>ORDER BY</codeph> clause in memory whenever practical. In
      a cluster of N DataNodes, each node sorts roughly 1/Nth of the result set, the exact proportion varying
      depending on how the data matching the query is distributed in HDFS.
    </p>

    <p>
      If the size of the sorted intermediate result set on any DataNode would cause the query to exceed the Impala
      memory limit, Impala sorts as much as practical in memory, then writes partially sorted data to disk. (This
      technique is known in industry terminology as <q>external sorting</q> and <q>spilling to disk</q>.) As each
      8 MB batch of data is written to disk, Impala frees the corresponding memory to sort a new 8 MB batch of
      data. When all the data has been processed, a final merge sort operation is performed to correctly order the
      in-memory and on-disk results as the result set is transmitted back to the coordinator node. When external
      sorting becomes necessary, Impala requires approximately 60 MB of RAM at a minimum for the buffers needed to
      read, write, and sort the intermediate results. If more RAM is available on the DataNode, Impala will use
      the additional RAM to minimize the amount of disk I/O for sorting.
    </p>

    <p>
      This external sort technique is used as appropriate on each DataNode (possibly including the coordinator
      node) to sort the portion of the result set that is processed on that node. When the sorted intermediate
      results are sent back to the coordinator node to produce the final result set, the coordinator node uses a
      merge sort technique to produce a final sorted result set without using any extra resources on the
      coordinator node.
    </p>

    <p rev="obwl">
      <b>Configuration for disk usage:</b>
    </p>

    <p rev="obwl" conref="../shared/impala_common.xml#common/order_by_scratch_dir"/>

<!-- Here is actually the more logical place to collect all those examples, move them from SELECT and cross-reference to here. -->

<!--     <p rev="obwl" conref="../shared/impala_common.xml#common/restrictions_blurb"/> -->

    <p rev="obwl" conref="../shared/impala_common.xml#common/insert_sort_blurb"/>

    <p rev="obwl" conref="../shared/impala_common.xml#common/order_by_view_restriction"/>

    <p>
      With the lifting of the requirement to include a <codeph>LIMIT</codeph> clause in every <codeph>ORDER
      BY</codeph> query (in Impala 1.4 and higher):
    </p>

    <ul>
      <li>
        <p>
          Now the use of scratch disk space raises the possibility of an <q>out of disk space</q> error on a
          particular DataNode, as opposed to the previous possibility of an <q>out of memory</q> error. Make sure
          to keep at least 1 GB free on the filesystem used for temporary sorting work.
        </p>
      </li>

      <li>
        <p>
          The query options
          <xref href="impala_default_order_by_limit.xml#default_order_by_limit">DEFAULT_ORDER_BY_LIMIT</xref> and
          <xref href="impala_abort_on_default_limit_exceeded.xml#abort_on_default_limit_exceeded">ABORT_ON_DEFAULT_LIMIT_EXCEEDED</xref>,
          which formerly controlled the behavior of <codeph>ORDER BY</codeph> queries with no limit specified, are
          now ignored.
        </p>
      </li>
    </ul>

    <p rev="obwl" conref="../shared/impala_common.xml#common/null_sorting_change"/>
<codeblock>[localhost:21000] > create table numbers (x int);
[localhost:21000] > insert into numbers values (1), (null), (2), (null), (3);
[localhost:21000] > select x from numbers order by x nulls first;
+------+
| x    |
+------+
| NULL |
| NULL |
| 1    |
| 2    |
| 3    |
+------+
[localhost:21000] > select x from numbers order by x desc nulls first;
+------+
| x    |
+------+
| NULL |
| NULL |
| 3    |
| 2    |
| 1    |
+------+
[localhost:21000] > select x from numbers order by x nulls last;
+------+
| x    |
+------+
| 1    |
| 2    |
| 3    |
| NULL |
| NULL |
+------+
[localhost:21000] > select x from numbers order by x desc nulls last;
+------+
| x    |
+------+
| 3    |
| 2    |
| 1    |
| NULL |
| NULL |
+------+
</codeblock>

    <p rev="obwl" conref="../shared/impala_common.xml#common/related_info"/>

    <p rev="obwl">
      See <xref href="impala_select.xml#select"/> for further examples of queries with the <codeph>ORDER
      BY</codeph> clause.
    </p>

    <p>
      Analytic functions use the <codeph>ORDER BY</codeph> clause in a different context to define the sequence in
      which rows are analyzed. See <xref href="impala_analytic_functions.xml#analytic_functions"/> for details.
    </p>
  </conbody>
</concept>