Re: NULL input for array_agg()?

Hi,

During reviewing aggregates ORDER BY, I was reading spec and found
description like:

== snip ==

Of the rows in the aggregation, the following do not qualify:
— If DISTINCT is specified, then redundant duplicates.
— Every row in which the <value expression> evaluates to the null value.

== /snip ==

... and array_agg() is among the functions that description above
refers to. So I wonder if array_agg doesn't accept NULL input (with
STRICT trans function). Did we discussed about this issue when
implementing it for 8.4?

Regards,

--
Hitoshi Harada

>>>>> "Hitoshi" == Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com> writes:

Hitoshi> Hi, During reviewing aggregates ORDER BY, I was reading spec
Hitoshi> and found description like:

Hitoshi> == snip ==

Hitoshi> Of the rows in the aggregation, the following do not qualify:
Hitoshi> — If DISTINCT is specified, then redundant duplicates.
Hitoshi> — Every row in which the <value expression> evaluates to the null value.

Hitoshi> == /snip ==

Where did you find that?

The SQL2008 last-call draft says this:

4) If <general set function> is specified, then:
a) Let TX be the single-column table that is the result of applying
the <value expression> to each row of T1 and eliminating null
values. If one or more null values are eliminated, then a
completion condition is raised: warning -- null value eliminated
in set function.
b) Case:
i) If DISTINCT is specified, then let TXA be the result of
eliminating redundant duplicate values from TX, using the
comparison rules specified in Subclause 8.2, "<comparison
predicate>", to identify the redundant duplicate values.
ii) Otherwise, let TXA be TX.

[more subclauses of rule (4) snipped as irrelevant]

8) If <array aggregate function> is specified, then:

a) If <sort specification list> is specified, then let K be the
number of <sort key>s; otherwise, let K be 0 (zero).

b) Let TXA be the table of K+1 columns obtained by applying the
<value expression> immediately contained in the <array aggregate
function> to each row of T1 to obtain the first column of TXA,
and, for all i, 1 (one) i K, applying the <value expression>
simply contained in the i-th <sort key> to each row of T1 to
obtain the (i+1)-th column of TXA.

c) Let TXA be ordered according to the values of the <sort key>s
found in the second through (K+1)-th columns of TXA. If K is 0
(zero), then the ordering of TXA is implementation-dependent.

d) Let N be the number of rows in TXA.

e) If N is greater than IDMC, then an exception condition is
raised: data exception -- array data, right truncation.

f) Let Ri, 1 (one) i N, be the rows of TXA according to the
ordering of TXA.

g) Case:
i) If TXA is empty, then the result of <array aggregate
function> is the null value.
ii) Otherwise, the result of <array aggregate function> is an
array of N elements such that for all i, 1 (one) i N, the
value of the i-th element is the value of the first column
of Ri.

NOTE 267 -- Null values are not eliminated when computing <array
aggregate function>. This, plus the optional <sort specification
list>, sets <array aggregate function> apart from <general set
function>s.

array_agg is an <array aggregate function> (in fact the only such),
whereas <general set function> includes almost all the other single-arg
aggregates (avg, min, max, etc.)

--
Andrew (irc:RhodiumToad)

2009/11/15 Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>:
>>>>>> "Hitoshi" == Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com> writes:
>
>  Hitoshi> Hi, During reviewing aggregates ORDER BY, I was reading spec
>  Hitoshi> and found description like:
>
>  Hitoshi> == snip ==
>
>  Hitoshi> Of the rows in the aggregation, the following do not qualify:
>  Hitoshi> — If DISTINCT is specified, then redundant duplicates.
>  Hitoshi> — Every row in which the <value expression> evaluates to the null value.
>
>  Hitoshi> == /snip ==
>
> Where did you find that?

In 4.15.4 Aggregate functions. But your snip clarified <array
aggregate function> is special case.

> NOTE 267 -- Null values are not eliminated when computing <array
> aggregate function>. This, plus the optional <sort specification
> list>, sets <array aggregate function> apart from <general set
> function>s.

Regards,

--
Hitoshi Harada