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documentation for v6.4. Bigger updates to the installation instructions (install and config).
108 lines
3.3 KiB
Plaintext
108 lines
3.3 KiB
Plaintext
<Chapter Id="xaggr">
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<Title>Extending <Acronym>SQL</Acronym>: Aggregates</Title>
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<Para>
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Aggregates in <ProductName>Postgres</ProductName>
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are expressed in terms of state
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transition functions. That is, an aggregate can be
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defined in terms of state that is modified whenever an
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instance is processed. Some state functions look at a
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particular value in the instance when computing the new
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state (<Acronym>sfunc1</Acronym> in the
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create aggregate syntax) while
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others only keep track of their own internal state
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(<Acronym>sfunc2</Acronym>).
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If we define an aggregate that uses only
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<Acronym>sfunc1</Acronym>, we
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define 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. We will use the
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<Acronym>int4pl</Acronym> that is
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built into <ProductName>Postgres</ProductName>
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to perform this addition.
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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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stype1 = complex,
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initcond1 = '(0,0)'
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);
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SELECT complex_sum(a) FROM test_complex;
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+------------+
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|complex_sum |
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+------------+
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|(34,53.9) |
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+------------+
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</ProgramListing>
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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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CREATE AGGREGATE my_count (
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sfunc2 = int4inc, -- add one
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basetype = int4,
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stype2 = int4,
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initcond2 = '0'
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);
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SELECT my_count(*) as emp_count from EMP;
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+----------+
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|emp_count |
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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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CREATE AGGREGATE my_average (
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sfunc1 = int4pl, -- sum
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basetype = int4,
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stype1 = int4,
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sfunc2 = int4inc, -- count
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stype2 = int4,
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finalfunc = int4div, -- division
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initcond1 = '0',
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initcond2 = '0'
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);
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SELECT my_average(salary) as emp_average FROM EMP;
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+------------+
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|emp_average |
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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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</Chapter>
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