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docs: Improve window function docs

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Specifically, the behavior of general-purpose and statistical aggregates
as window functions was not clearly documented, and terms were
inconsistently used.  Also add docs about the difference between
cume_dist and percent_rank, rather than just the formulas.

Discussion: 20170406214918.GA5757@momjian.us
This commit is contained in:
Bruce Momjian 2017-04-11 10:47:40 -04:00
parent a4777f3556
commit 1c1a4726eb
3 changed files with 57 additions and 45 deletions
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31
doc/src/sgml/advanced.sgml
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@ -328,8 +328,8 @@ COMMIT;
A <firstterm>window function</> performs a calculation across a set of
table rows that are somehow related to the current row. This is comparable
to the type of calculation that can be done with an aggregate function.
But unlike regular aggregate functions, use of a window function does not
cause rows to become grouped into a single output row &mdash; the
However, window functions do not cause rows to become grouped into a single
output row like non-window aggregate calls would. Instead, the
rows retain their separate identities. Behind the scenes, the window
function is able to access more than just the current row of the query
result.
@ -363,20 +363,19 @@ SELECT depname, empno, salary, avg(salary) OVER (PARTITION BY depname) FROM emps
<structname>empsalary</>, and there is one output row for each row in the
table. The fourth column represents an average taken across all the table
rows that have the same <structfield>depname</> value as the current row.
(This actually is the same function as the regular <function>avg</>
aggregate function, but the <literal>OVER</> clause causes it to be
treated as a window function and computed across an appropriate set of
rows.)
(This actually is the same function as the non-window <function>avg</>
aggregate, but the <literal>OVER</> clause causes it to be
treated as a window function and computed across the window frame.)
</para>
<para>
A window function call always contains an <literal>OVER</> clause
directly following the window function's name and argument(s). This is what
syntactically distinguishes it from a regular function or aggregate
function. The <literal>OVER</> clause determines exactly how the
syntactically distinguishes it from a normal function or non-window
aggregate. The <literal>OVER</> clause determines exactly how the
rows of the query are split up for processing by the window function.
The <literal>PARTITION BY</> list within <literal>OVER</> specifies
dividing the rows into groups, or partitions, that share the same
The <literal>PARTITION BY</> clause within <literal>OVER</>
divides the rows into groups, or partitions, that share the same
values of the <literal>PARTITION BY</> expression(s). For each row,
the window function is computed across the rows that fall into the
same partition as the current row.
@ -411,8 +410,8 @@ FROM empsalary;
</screen>
As shown here, the <function>rank</> function produces a numerical rank
within the current row's partition for each distinct <literal>ORDER BY</>
value, in the order defined by the <literal>ORDER BY</> clause.
for each distinct <literal>ORDER BY</> value in the current row's
partition, using the order defined by the <literal>ORDER BY</> clause.
<function>rank</> needs no explicit parameter, because its behavior
is entirely determined by the <literal>OVER</> clause.
</para>
@ -424,20 +423,20 @@ FROM empsalary;
if any. For example, a row removed because it does not meet the
<literal>WHERE</> condition is not seen by any window function.
A query can contain multiple window functions that slice up the data
in different ways by means of different <literal>OVER</> clauses, but
in different ways using different <literal>OVER</> clauses, but
they all act on the same collection of rows defined by this virtual table.
</para>
<para>
We already saw that <literal>ORDER BY</> can be omitted if the ordering
of rows is not important. It is also possible to omit <literal>PARTITION
BY</>, in which case there is just one partition containing all the rows.
BY</>, in which case there is a single partition containing all rows.
</para>
<para>
There is another important concept associated with window functions:
for each row, there is a set of rows within its partition called its
<firstterm>window frame</>. Many (but not all) window functions act only
<firstterm>window frame</>. Some window functions act only
on the rows of the window frame, rather than of the whole partition.
By default, if <literal>ORDER BY</> is supplied then the frame consists of
all rows from the start of the partition up through the current row, plus
@ -515,7 +514,7 @@ SELECT salary, sum(salary) OVER (ORDER BY salary) FROM empsalary;
elsewhere, such as in <literal>GROUP BY</>, <literal>HAVING</>
and <literal>WHERE</literal> clauses. This is because they logically
execute after the processing of those clauses. Also, window functions
execute after regular aggregate functions. This means it is valid to
execute after non-window aggregate functions. This means it is valid to
include an aggregate function call in the arguments of a window function,
but not vice versa.
</para>

44
doc/src/sgml/func.sgml
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@ -13166,13 +13166,14 @@ NULL baz</literallayout>(3 rows)</entry>
<para>
<firstterm>Aggregate functions</firstterm> compute a single result
from a set of input values. The built-in normal aggregate functions
are listed in
<xref linkend="functions-aggregate-table"> and
<xref linkend="functions-aggregate-statistics-table">.
The built-in ordered-set aggregate functions
are listed in <xref linkend="functions-orderedset-table"> and
<xref linkend="functions-hypothetical-table">. Grouping operations,
from a set of input values. The built-in general-purpose aggregate
functions are listed in <xref linkend="functions-aggregate-table">
and statistical aggregates in <xref
linkend="functions-aggregate-statistics-table">.
The built-in within-group ordered-set aggregate functions
are listed in <xref linkend="functions-orderedset-table">
while the built-in within-group hypothetical-set ones are in <xref
linkend="functions-hypothetical-table">. Grouping operations,
which are closely related to aggregate functions, are listed in
<xref linkend="functions-grouping-table">.
The special syntax considerations for aggregate
@ -14413,17 +14414,19 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
<para>
The built-in window functions are listed in
<xref linkend="functions-window-table">. Note that these functions
<emphasis>must</> be invoked using window function syntax; that is an
<emphasis>must</> be invoked using window function syntax, i.e., an
<literal>OVER</> clause is required.
</para>
<para>
In addition to these functions, any built-in or user-defined normal
aggregate function (but not ordered-set or hypothetical-set aggregates)
In addition to these functions, any built-in or user-defined
general-purpose or statistical
aggregate (i.e., not ordered-set or hypothetical-set aggregates)
can be used as a window function; see
<xref linkend="functions-aggregate"> for a list of the built-in aggregates.
Aggregate functions act as window functions only when an <literal>OVER</>
clause follows the call; otherwise they act as regular aggregates.
clause follows the call; otherwise they act as non-window aggregates
and return a single row for the entire set.
</para>
<table id="functions-window-table">
@ -14488,7 +14491,7 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
<entry>
<type>double precision</type>
</entry>
<entry>relative rank of the current row: (<function>rank</> - 1) / (total rows - 1)</entry>
<entry>relative rank of the current row: (<function>rank</> - 1) / (total partition rows - 1)</entry>
</row>
<row>
@ -14501,7 +14504,7 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
<entry>
<type>double precision</type>
</entry>
<entry>relative rank of the current row: (number of rows preceding or peer with current row) / (total rows)</entry>
<entry>cumulative distribution: (number of partition rows preceding or peer with current row) / total partition rows</entry>
</row>
<row>
@ -14634,9 +14637,10 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
All of the functions listed in
<xref linkend="functions-window-table"> depend on the sort ordering
specified by the <literal>ORDER BY</> clause of the associated window
definition. Rows that are not distinct in the <literal>ORDER BY</>
ordering are said to be <firstterm>peers</>; the four ranking functions
are defined so that they give the same answer for any two peer rows.
definition. Rows that are not distinct when considering only the
<literal>ORDER BY</> columns are said to be <firstterm>peers</>.
The four ranking functions (including <function>cume_dist</>) are
defined so that they give the same answer for all peer rows.
</para>
<para>
@ -14679,6 +14683,14 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
</para>
</note>
<para>
<function>cume_dist</> computes the fraction of partition rows that
are less than or equal to the current row and its peers, while
<function>percent_rank</> computes the fraction of partition rows that
are less than the current row, assuming the current row does not exist
in the partition.
</para>
</sect1>
<sect1 id="functions-subquery">

27
doc/src/sgml/syntax.sgml
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@ -1664,7 +1664,8 @@ SELECT string_agg(a ORDER BY a, ',') FROM table; -- incorrect
<para>
Placing <literal>ORDER BY</> within the aggregate's regular argument
list, as described so far, is used when ordering the input rows for
a <quote>normal</> aggregate for which ordering is optional. There is a
general-purpose and statistical aggregates, for which ordering is
optional. There is a
subclass of aggregate functions called <firstterm>ordered-set
aggregates</> for which an <replaceable>order_by_clause</replaceable>
is <emphasis>required</>, usually because the aggregate's computation is
@ -1675,7 +1676,7 @@ SELECT string_agg(a ORDER BY a, ',') FROM table; -- incorrect
inside <literal>WITHIN GROUP (...)</>, as shown in the final syntax
alternative above. The expressions in
the <replaceable>order_by_clause</replaceable> are evaluated once per
input row just like normal aggregate arguments, sorted as per
input row just like regular aggregate arguments, sorted as per
the <replaceable>order_by_clause</replaceable>'s requirements, and fed
to the aggregate function as input arguments. (This is unlike the case
for a non-<literal>WITHIN GROUP</> <replaceable>order_by_clause</>,
@ -1683,7 +1684,7 @@ SELECT string_agg(a ORDER BY a, ',') FROM table; -- incorrect
argument expressions preceding <literal>WITHIN GROUP</>, if any, are
called <firstterm>direct arguments</> to distinguish them from
the <firstterm>aggregated arguments</> listed in
the <replaceable>order_by_clause</replaceable>. Unlike normal aggregate
the <replaceable>order_by_clause</replaceable>. Unlike regular aggregate
arguments, direct arguments are evaluated only once per aggregate call,
not once per input row. This means that they can contain variables only
if those variables are grouped by <literal>GROUP BY</>; this restriction
@ -1779,10 +1780,10 @@ FROM generate_series(1,10) AS s(i);
<para>
A <firstterm>window function call</firstterm> represents the application
of an aggregate-like function over some portion of the rows selected
by a query. Unlike regular aggregate function calls, this is not tied
by a query. Unlike non-window aggregate calls, this is not tied
to grouping of the selected rows into a single output row &mdash; each
row remains separate in the query output. However the window function
is able to scan all the rows that would be part of the current row's
has access to all the rows that would be part of the current row's
group according to the grouping specification (<literal>PARTITION BY</>
list) of the window function call.
The syntax of a window function call is one of the following:
@ -1831,20 +1832,20 @@ UNBOUNDED FOLLOWING
named window in the <literal>WINDOW</literal> clause; see the
<xref linkend="sql-select"> reference page for details. It's worth
pointing out that <literal>OVER wname</> is not exactly equivalent to
<literal>OVER (wname)</>; the latter implies copying and modifying the
<literal>OVER (wname ...)</>; the latter implies copying and modifying the
window definition, and will be rejected if the referenced window
specification includes a frame clause.
</para>
<para>
The <literal>PARTITION BY</> option groups the rows of the query into
The <literal>PARTITION BY</> clause groups the rows of the query into
<firstterm>partitions</>, which are processed separately by the window
function. <literal>PARTITION BY</> works similarly to a query-level
<literal>GROUP BY</> clause, except that its expressions are always just
expressions and cannot be output-column names or numbers.
Without <literal>PARTITION BY</>, all rows produced by the query are
treated as a single partition.
The <literal>ORDER BY</> option determines the order in which the rows
The <literal>ORDER BY</> clause determines the order in which the rows
of a partition are processed by the window function. It works similarly
to a query-level <literal>ORDER BY</> clause, but likewise cannot use
output-column names or numbers. Without <literal>ORDER BY</>, rows are
@ -1921,17 +1922,17 @@ UNBOUNDED FOLLOWING
<para>
The built-in window functions are described in <xref
linkend="functions-window-table">. Other window functions can be added by
the user. Also, any built-in or user-defined normal aggregate function
can be used as a window function. Ordered-set aggregates presently
cannot be used as window functions, however.
the user. Also, any built-in or user-defined general-purpose or
statistical aggregate can be used as a window function. (Ordered-set
and hypothetical-set aggregates cannot presently be used as window functions.)
</para>
<para>
The syntaxes using <literal>*</> are used for calling parameter-less
aggregate functions as window functions, for example
<literal>count(*) OVER (PARTITION BY x ORDER BY y)</>.
The asterisk (<literal>*</>) is customarily not used for non-aggregate window functions.
Aggregate window functions, unlike normal aggregate functions, do not
The asterisk (<literal>*</>) is customarily not used for
window-specific functions. Window-specific functions do not
allow <literal>DISTINCT</> or <literal>ORDER BY</> to be used within the
function argument list.
</para>