Change cross reference to CREATE AGGREGATE to a citation, since the
Programmer's Guide does not have access to the reference page. Normalize markup.
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Thomas G. Lockhart
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<Chapter Id="xaggr">
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<Title>Extending <Acronym>SQL</Acronym>: Aggregates</Title>
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<chapter id="xaggr">
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<title>Extending <acronym>SQL</acronym>: Aggregates</title>
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<Para>
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Aggregate functions in <ProductName>Postgres</ProductName>
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are expressed as <firstterm>state values</firstterm>
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and <firstterm>state transition functions</firstterm>.
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That is, an aggregate can be
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defined in terms of state that is modified whenever an
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input item is processed. To define a new aggregate
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function, one selects a datatype for the state value,
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an initial value for the state, and a state transition
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function. The state transition function is just an
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ordinary function that could also be used outside the
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context of the aggregate.
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</Para>
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<para>
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Aggregate functions in <productname>Postgres</productname>
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are expressed as <firstterm>state values</firstterm>
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and <firstterm>state transition functions</firstterm>.
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That is, an aggregate can be
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defined in terms of state that is modified whenever an
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input item is processed. To define a new aggregate
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function, one selects a datatype for the state value,
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an initial value for the state, and a state transition
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function. The state transition function is just an
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ordinary function that could also be used outside the
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context of the aggregate.
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</para>
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<Para>
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Actually, in order to make it easier to construct useful
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aggregates from existing functions, an aggregate can have
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one or two separate state values, one or two transition
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functions to update those state values, and a
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<firstterm>final function</firstterm> that computes the
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actual aggregate result from the ending state values.
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</Para>
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<para>
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Actually, in order to make it easier to construct useful
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aggregates from existing functions, an aggregate can have
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one or two separate state values, one or two transition
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functions to update those state values, and a
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<firstterm>final function</firstterm> that computes the
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actual aggregate result from the ending state values.
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</para>
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<Para>
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Thus there can be as many as four datatypes involved:
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the type of the input data items, the type of the aggregate's
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result, and the types of the two state values. Only the
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input and result datatypes are seen by a user of the aggregate.
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</Para>
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<para>
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Thus there can be as many as four datatypes involved:
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the type of the input data items, the type of the aggregate's
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result, and the types of the two state values. Only the
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input and result datatypes are seen by a user of the aggregate.
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</para>
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<Para>
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Some state transition functions need to look at each successive
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input to compute the next state value, while others ignore the
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specific input value and simply update their internal state.
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(The most useful example of the second kind is a running count
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of the number of input items.) The <ProductName>Postgres</ProductName>
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aggregate machinery defines <Acronym>sfunc1</Acronym> for
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an aggregate as a function that is passed both the old state
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value and the current input value, while <Acronym>sfunc2</Acronym>
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is a function that is passed only the old state value.
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</Para>
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<para>
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Some state transition functions need to look at each successive
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input to compute the next state value, while others ignore the
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specific input value and simply update their internal state.
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(The most useful example of the second kind is a running count
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of the number of input items.) The <productname>Postgres</productname>
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aggregate machinery defines <acronym>sfunc1</acronym> for
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an aggregate as a function that is passed both the old state
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value and the current input value, while <acronym>sfunc2</acronym>
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is a function that is passed only the old state value.
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</para>
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<Para>
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If we define an aggregate that uses only <Acronym>sfunc1</Acronym>,
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we have an aggregate that computes a running function of
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the attribute values from each instance. "Sum" is an
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example of this kind of aggregate. "Sum" starts at
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zero and always adds the current instance's value to
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its running total. For example, if we want to make a Sum
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aggregate to work on a datatype for complex numbers,
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we only need the addition function for that datatype.
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The aggregate definition is:
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<ProgramListing>
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<para>
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If we define an aggregate that uses only <acronym>sfunc1</acronym>,
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we have an aggregate that computes a running function of
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the attribute values from each instance. "Sum" is an
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example of this kind of aggregate. "Sum" starts at
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zero and always adds the current instance's value to
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its running total. For example, if we want to make a Sum
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aggregate to work on a datatype for complex numbers,
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we only need the addition function for that datatype.
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The aggregate definition is:
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<programlisting>
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CREATE AGGREGATE complex_sum (
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sfunc1 = complex_add,
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basetype = complex,
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@ -69,27 +69,27 @@ SELECT complex_sum(a) FROM test_complex;
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+------------+
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|(34,53.9) |
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+------------+
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</ProgramListing>
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</programlisting>
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(In practice, we'd just name the aggregate "sum", and rely on
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<ProductName>Postgres</ProductName> to figure out which kind
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of sum to apply to a complex column.)
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</Para>
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(In practice, we'd just name the aggregate "sum", and rely on
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<productname>Postgres</productname> to figure out which kind
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of sum to apply to a complex column.)
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</para>
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<Para>
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If we define only <Acronym>sfunc2</Acronym>, we are
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specifying an aggregate
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that computes a running function that is independent of
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the attribute values from each instance.
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"Count" is the most common example of this kind of
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aggregate. "Count" starts at zero and adds one to its
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running total for each instance, ignoring the instance
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value. Here, we use the built-in
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<Acronym>int4inc</Acronym> routine to do
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the work for us. This routine increments (adds one to)
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its argument.
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<ProgramListing>
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<para>
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If we define only <acronym>sfunc2</acronym>, we are
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specifying an aggregate
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that computes a running function that is independent of
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the attribute values from each instance.
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"Count" is the most common example of this kind of
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aggregate. "Count" starts at zero and adds one to its
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running total for each instance, ignoring the instance
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value. Here, we use the built-in
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<acronym>int4inc</acronym> routine to do
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the work for us. This routine increments (adds one to)
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its argument.
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<programlisting>
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CREATE AGGREGATE my_count (
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sfunc2 = int4inc, -- add one
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basetype = int4,
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@ -104,21 +104,22 @@ SELECT my_count(*) as emp_count from EMP;
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+----------+
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|5 |
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+----------+
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</ProgramListing>
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</Para>
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<Para>
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"Average" is an example of an aggregate that requires
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both a function to compute the running sum and a function
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to compute the running count. When all of the
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instances have been processed, the final answer for the
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aggregate is the running sum divided by the running
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count. We use the <Acronym>int4pl</Acronym> and <Acronym>int4inc</Acronym> routines we used
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before as well as the <ProductName>Postgres</ProductName> integer division
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routine, <Acronym>int4div</Acronym>, to compute the division of the sum by
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the count.
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<ProgramListing>
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</programlisting>
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</para>
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<para>
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"Average" is an example of an aggregate that requires
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both a function to compute the running sum and a function
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to compute the running count. When all of the
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instances have been processed, the final answer for the
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aggregate is the running sum divided by the running
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count. We use the <acronym>int4pl</acronym> and
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<acronym>int4inc</acronym> routines we used
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before as well as the <productname>Postgres</productname> integer division
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routine, <acronym>int4div</acronym>, to compute the division of the sum by
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the count.
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<programlisting>
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CREATE AGGREGATE my_average (
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sfunc1 = int4pl, -- sum
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basetype = int4,
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@ -137,12 +138,34 @@ SELECT my_average(salary) as emp_average FROM EMP;
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+------------+
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|1640 |
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+------------+
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</ProgramListing>
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</Para>
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</programlisting>
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</para>
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<Para>
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For further details see
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<xref endterm="sql-createaggregate-title"
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linkend="sql-createaggregate-title">.
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</Para>
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</Chapter>
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<para>
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For further details see
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<!--
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Not available in the Programmer's Guide
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<xref endterm="sql-createaggregate-title"
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linkend="sql-createaggregate-title">.
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-->
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<command>CREATE AGGREGATE</command> in
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<citetitle>The PostgreSQL User's Guide</citetitle>.
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</para>
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</chapter>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode:sgml
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sgml-omittag:nil
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"./reference.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:("/usr/lib/sgml/catalog")
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sgml-local-ecat-files:nil
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End:
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-->
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