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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] > create database limits; |
| [localhost:21000] > use limits; |
| [localhost:21000] > create table numbers (x int); |
| [localhost:21000] > insert into numbers values (1), (3), (4), (5), (2); |
| Inserted 5 rows in 1.34s |
| [localhost:21000] > select x from numbers limit 100; |
| +---+ |
| | x | |
| +---+ |
| | 1 | |
| | 3 | |
| | 4 | |
| | 5 | |
| | 2 | |
| +---+ |
| Returned 5 row(s) in 0.26s |
| [localhost:21000] > select x from numbers limit 3; |
| +---+ |
| | x | |
| +---+ |
| | 1 | |
| | 3 | |
| | 4 | |
| +---+ |
| Returned 3 row(s) in 0.27s |
| [localhost:21000] > select x from numbers where x > 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] > select x as "Top 3" from numbers order by x desc limit 3; |
| +-------+ |
| | top 3 | |
| +-------+ |
| | 5 | |
| | 4 | |
| | 3 | |
| +-------+ |
| [localhost:21000] > 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> |