impala/docs/topics/impala_limit.xml

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

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

  <conbody>

    <p>
      The <codeph>LIMIT</codeph> clause in a <codeph>SELECT</codeph> query sets a maximum number of rows for the
      result set. Pre-selecting the maximum size of the result set helps Impala to optimize memory usage while
      processing a distributed query.
    </p>

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

<codeblock>LIMIT <varname>constant_integer_expression</varname></codeblock>

    <p>
      The argument to the <codeph>LIMIT</codeph> clause must evaluate to a constant value. It can be a numeric
      literal, or another kind of numeric expression involving operators, casts, and function return values. You
      cannot refer to a column or use a subquery.
    </p>

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

    <p>
      This clause is useful in contexts such as:
    </p>

    <ul>
      <li>
        To return exactly N items from a top-N query, such as the 10 highest-rated items in a shopping category or
        the 50 hostnames that refer the most traffic to a web site.
      </li>

      <li>
        To demonstrate some sample values from a table or a particular query. (To display some arbitrary items, use
        a query with no <codeph>ORDER BY</codeph> clause. An <codeph>ORDER BY</codeph> clause causes additional
        memory and/or disk usage during the query.)
      </li>

      <li>
        To keep queries from returning huge result sets by accident if a table is larger than expected, or a
        <codeph>WHERE</codeph> clause matches more rows than expected.
      </li>
    </ul>

    <p rev="1.2.1">
      Originally, the value for the <codeph>LIMIT</codeph> clause had to be a numeric literal. In Impala 1.2.1 and
      higher, it can be a numeric expression.
    </p>

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

    <p>
      See <xref href="impala_order_by.xml#order_by"/> for details.
    </p>

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

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

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

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

    <p>
      The following example shows how the <codeph>LIMIT</codeph> clause caps the size of the result set, with the
      limit being applied after any other clauses such as <codeph>WHERE</codeph>.
    </p>

<codeblock>[localhost:21000] &gt; create database limits;
[localhost:21000] &gt; use limits;
[localhost:21000] &gt; create table numbers (x int);
[localhost:21000] &gt; insert into numbers values (1), (3), (4), (5), (2);
Inserted 5 rows in 1.34s
[localhost:21000] &gt; select x from numbers limit 100;
+---+
| x |
+---+
| 1 |
| 3 |
| 4 |
| 5 |
| 2 |
+---+
Returned 5 row(s) in 0.26s
[localhost:21000] &gt; select x from numbers limit 3;
+---+
| x |
+---+
| 1 |
| 3 |
| 4 |
+---+
Returned 3 row(s) in 0.27s
[localhost:21000] &gt; select x from numbers where x &gt; 2 limit 2;
+---+
| x |
+---+
| 3 |
| 4 |
+---+
Returned 2 row(s) in 0.27s</codeblock>

    <p>
      For top-N and bottom-N queries, you use the <codeph>ORDER BY</codeph> and <codeph>LIMIT</codeph> clauses
      together:
    </p>

<codeblock rev="obwl">[localhost:21000] &gt; select x as "Top 3" from numbers order by x desc limit 3;
+-------+
| top 3 |
+-------+
| 5     |
| 4     |
| 3     |
+-------+
[localhost:21000] &gt; select x as "Bottom 3" from numbers order by x limit 3;
+----------+
| bottom 3 |
+----------+
| 1        |
| 2        |
| 3        |
+----------+
</codeblock>

    <p>
      You can use constant values besides integer literals as the <codeph>LIMIT</codeph> argument:
    </p>

<codeblock>-- Other expressions that yield constant integer values work too.
SELECT x FROM t1 LIMIT 1e6;                        -- Limit is one million.
SELECT x FROM t1 LIMIT length('hello world');      -- Limit is 11.
SELECT x FROM t1 LIMIT 2+2;                        -- Limit is 4.
SELECT x FROM t1 LIMIT cast(truncate(9.9) AS INT); -- Limit is 9.
</codeblock>
  </conbody>
</concept>