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Support window functions a la SQL:2008.
Hitoshi Harada, with some kibitzing from Heikki and Tom.
This commit is contained in:
Tom Lane
@@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.463 2008/12/19 16:25:16 petere Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.464 2008/12/28 18:53:53 tgl Exp $ -->
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<chapter id="functions">
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<title>Functions and Operators</title>
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@@ -10149,6 +10149,278 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
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</sect1>
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<sect1 id="functions-window">
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<title>Window Functions</title>
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<indexterm zone="functions-window">
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<primary>window function</primary>
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<secondary>built-in</secondary>
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</indexterm>
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<para>
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<firstterm>Window functions</firstterm> provide the ability to perform
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calculations across sets of rows that are related to the current query
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row. For information about this feature see
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<xref linkend="tutorial-window"> and
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<xref linkend="syntax-window-functions">.
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</para>
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<para>
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The built-in window functions are listed in
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<xref linkend="functions-window-table">. Note that these functions
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<emphasis>must</> be invoked using window function syntax; that is an
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<literal>OVER</> clause is required.
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</para>
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<para>
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In addition to these functions, any built-in or user-defined aggregate
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function can be used as a window function (see
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<xref linkend="functions-aggregate"> for a list of the built-in aggregates).
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Aggregate functions act as window functions only when an <literal>OVER</>
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clause follows the call; otherwise they act as regular aggregates.
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</para>
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<table id="functions-window-table">
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<title>General-Purpose Window Functions</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Return Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>
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<indexterm>
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<primary>row_number</primary>
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</indexterm>
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<function>row_number()</function>
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</entry>
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<entry>
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<type>bigint</type>
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</entry>
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<entry>number of the current row within its partition, counting from 1</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>rank</primary>
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</indexterm>
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<function>rank()</function>
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</entry>
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<entry>
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<type>bigint</type>
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</entry>
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<entry>rank of the current row with gaps; same as <function>row_number</> of its first peer</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>dense_rank</primary>
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</indexterm>
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<function>dense_rank()</function>
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</entry>
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<entry>
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<type>bigint</type>
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</entry>
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<entry>rank of the current row without gaps; this function counts peer groups</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>percent_rank</primary>
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</indexterm>
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<function>percent_rank()</function>
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</entry>
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<entry>
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<type>double precision</type>
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</entry>
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<entry>relative rank of the current row: (<function>rank</> - 1) / (total rows - 1)</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>cume_dist</primary>
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</indexterm>
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<function>cume_dist()</function>
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</entry>
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<entry>
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<type>double precision</type>
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</entry>
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<entry>relative rank of the current row: (number of rows preceding or peer with current row) / (total rows)</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>ntile</primary>
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</indexterm>
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<function>ntile(<replaceable class="parameter">num_buckets</replaceable> <type>integer</>)</function>
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</entry>
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<entry>
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<type>integer</type>
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</entry>
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<entry>integer ranging from 1 to the argument value, dividing the
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partition as equally as possible</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>lag</primary>
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</indexterm>
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<function>
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lag(<replaceable class="parameter">value</replaceable> <type>any</>
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[, <replaceable class="parameter">offset</replaceable> <type>integer</>
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[, <replaceable class="parameter">default</replaceable> <type>any</> ]])
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</function>
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</entry>
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<entry>
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<type>same type as <replaceable class="parameter">value</replaceable></type>
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</entry>
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<entry>
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returns <replaceable class="parameter">value</replaceable> evaluated at
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the row that is <replaceable class="parameter">offset</replaceable>
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rows before the current row within the partition; if there is no such
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row, instead return <replaceable class="parameter">default</replaceable>.
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Both <replaceable class="parameter">offset</replaceable> and
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<replaceable class="parameter">default</replaceable> are evaluated
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with respect to the current row. If omitted,
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<replaceable class="parameter">offset</replaceable> defaults to 1 and
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<replaceable class="parameter">default</replaceable> to null
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</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>lead</primary>
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</indexterm>
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<function>
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lead(<replaceable class="parameter">value</replaceable> <type>any</>
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[, <replaceable class="parameter">offset</replaceable> <type>integer</>
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[, <replaceable class="parameter">default</replaceable> <type>any</> ]])
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</function>
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</entry>
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<entry>
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<type>same type as <replaceable class="parameter">value</replaceable></type>
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</entry>
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<entry>
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returns <replaceable class="parameter">value</replaceable> evaluated at
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the row that is <replaceable class="parameter">offset</replaceable>
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rows after the current row within the partition; if there is no such
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row, instead return <replaceable class="parameter">default</replaceable>.
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Both <replaceable class="parameter">offset</replaceable> and
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<replaceable class="parameter">default</replaceable> are evaluated
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with respect to the current row. If omitted,
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<replaceable class="parameter">offset</replaceable> defaults to 1 and
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<replaceable class="parameter">default</replaceable> to null
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</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>first_value</primary>
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</indexterm>
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<function>first_value(<replaceable class="parameter">value</replaceable> <type>any</>)</function>
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</entry>
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<entry>
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<type>same type as <replaceable class="parameter">value</replaceable></type>
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</entry>
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<entry>
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returns <replaceable class="parameter">value</replaceable> evaluated
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at the row that is the first row of the window frame
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</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>last_value</primary>
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</indexterm>
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<function>last_value(<replaceable class="parameter">value</replaceable> <type>any</>)</function>
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</entry>
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<entry>
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<type>same type as <replaceable class="parameter">value</replaceable></type>
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</entry>
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<entry>
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returns <replaceable class="parameter">value</replaceable> evaluated
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at the row that is the last row of the window frame
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</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>nth_value</primary>
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</indexterm>
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<function>
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nth_value(<replaceable class="parameter">value</replaceable> <type>any</>, <replaceable class="parameter">nth</replaceable> <type>integer</>)
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</function>
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</entry>
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<entry>
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<type>same type as <replaceable class="parameter">value</replaceable></type>
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</entry>
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<entry>
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returns <replaceable class="parameter">value</replaceable> evaluated
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at the row that is the <replaceable class="parameter">nth</replaceable>
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row of the window frame (counting from 1); null if no such row
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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All of the functions listed in
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<xref linkend="functions-window-table"> depend on the sort ordering
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specified by the <literal>ORDER BY</> clause of the associated window
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definition. Rows that are not distinct in the <literal>ORDER BY</>
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ordering are said to be <firstterm>peers</>; the four ranking functions
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are defined so that they give the same answer for any two peer rows.
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</para>
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<para>
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Note that <function>first_value</>, <function>last_value</>, and
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<function>nth_value</> consider only the rows within the <quote>window
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frame</>, that is the rows from the start of the partition through the
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last peer of the current row. This is particularly likely to give
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unintuitive results for <function>last_value</>.
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</para>
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<para>
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When an aggregate function is used as a window function, it aggregates
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over the rows within the current row's window frame. To obtain
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aggregation over the whole partition, be sure to omit <literal>ORDER BY</>
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from the window definition. An aggregate used with <literal>ORDER BY</>
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produces a <quote>running sum</> type of behavior, which may or may not
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be what's wanted.
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</para>
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<note>
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<para>
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The SQL standard defines a <literal>RESPECT NULLS</> or
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<literal>IGNORE NULLS</> option for <function>lead</>, <function>lag</>,
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<function>first_value</>, <function>last_value</>, and
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<function>nth_value</>. This is not implemented in
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<productname>PostgreSQL</productname>: the behavior is always the
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same as the standard's default, namely <literal>RESPECT NULLS</>.
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Likewise, the standard's <literal>FROM FIRST</> or <literal>FROM LAST</>
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option for <function>nth_value</> is not implemented: only the
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default <literal>FROM FIRST</> behavior is supported.
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</para>
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</note>
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</sect1>
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<sect1 id="functions-subquery">
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<title>Subquery Expressions</title>
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Reference in New Issue
Block a user