diff options
| -rw-r--r-- | doc/src/sgml/xaggr.sgml | 13 |
1 files changed, 9 insertions, 4 deletions
diff --git a/doc/src/sgml/xaggr.sgml b/doc/src/sgml/xaggr.sgml index 1d691e7759d..82197e13816 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: @@ -106,9 +106,14 @@ CREATE AGGREGATE avg (float8) sfunc = float8_accum, stype = float8[], finalfunc = float8_avg, - initcond = '{0,0,0}' + initcond = '{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> |