diff options
| -rw-r--r-- | doc/src/sgml/xaggr.sgml | 16 |
1 files changed, 11 insertions, 5 deletions
diff --git a/doc/src/sgml/xaggr.sgml b/doc/src/sgml/xaggr.sgml index 9fc0722f454..811934bd75b 100644 --- a/doc/src/sgml/xaggr.sgml +++ b/doc/src/sgml/xaggr.sgml @@ -70,7 +70,7 @@ SELECT sum(a) FROM test_complex; expects <function>sum</function> to behave that way. We can do this simply by omitting the <literal>initcond</literal> phrase, so that the initial state condition is null. Ordinarily this would mean that the <literal>sfunc</literal> - would need to check for a null state-condition input, but for + would need to check for a null state-condition input. But for <function>sum</function> and some other simple aggregates like <function>max</> and <function>min</>, it is sufficient to insert the first nonnull input value into @@ -95,8 +95,8 @@ SELECT sum(a) FROM test_complex; It requires two pieces of running state: the sum of the inputs and the count of the number of inputs. The final result is obtained by dividing - these quantities. Average is typically implemented by using a - two-element array as the state value. For example, + these quantities. Average is typically implemented by using an + array as the state value. For example, the built-in implementation of <function>avg(float8)</function> looks like: @@ -109,6 +109,11 @@ CREATE AGGREGATE avg (float8) initcond = '{0,0,0}' ); </programlisting> + + (<function>float8_accum</> requires a three-element array, not just + two elements, because it accumulates the sum of squares as well as + the sum and count of the inputs. This is so that it can be used for + some other aggregates besides <function>avg</>.) </para> <para> @@ -174,12 +179,13 @@ if (AggCheckCallContext(fcinfo, NULL)) One reason for checking this is that when it is true for a transition function, the first input must be a temporary transition value and can therefore safely be modified - in-place rather than allocating a new copy. (This is the <emphasis>only</> + in-place rather than allocating a new copy. + See <literal>int8inc()</> for an example. + (This is the <emphasis>only</> case where it is safe for a function to modify a pass-by-reference input. In particular, aggregate final functions should not modify their inputs in any case, because in some cases they will be re-executed on the same final transition value.) - See <literal>int8inc()</> for an example. </para> <para> |