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 <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> First, data is sorted locally by each <codeph>impalad</codeph> daemon,
      then streamed to the coordinator daemon, which merges the sorted result
      sets. For distributed queries, this is a relatively expensive operation
      and 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>

    <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"/>
    <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>
      pseudo-column 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 executor Impala daemon 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 executor Impala
      daemons, each daemon 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 executor Impala
      daemon 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 Impala daemon,
      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 Impala
      daemon (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"/>

    <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 Impala daemon, 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>

    </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>