Re: UNNEST ... WITH ORDINALITY (AND POSSIBLY OTHER STUFF)

Note the standard also supports unnesting multiple arrays concurrently, the rule for handling arrays with different lengths is to use null padding of the shorter array.

SELECT * FROM
UNNEST( ARRAY[5,2,3,4],
ARRAY['hello', 'world'] )
WITH ORDINALITY AS t(a,b,i);

a b i
--- ---------- ------
5 'hello' 1
2 'world' 2
3 3
4 4
(4 rows)

To implement this it is not just substituting the existing unnest(anyarray) function in multiple times.

Regards,
Caleb

On Nov 19, 2010, at 4:50 AM, <pgsql-hackers-owner(at)postgresql(dot)org<mailto:pgsql-hackers-owner(at)postgresql(dot)org>> <pgsql-hackers-owner(at)postgresql(dot)org<mailto:pgsql-hackers-owner(at)postgresql(dot)org>> wrote:

From: David Fetter <david(at)fetter(dot)org<mailto:david(at)fetter(dot)org>>
Date: November 18, 2010 11:48:16 PM PST
To: Itagaki Takahiro <itagaki(dot)takahiro(at)gmail(dot)com<mailto:itagaki(dot)takahiro(at)gmail(dot)com>>
Cc: PG Hackers <pgsql-hackers(at)postgresql(dot)org<mailto:pgsql-hackers(at)postgresql(dot)org>>
Subject: Re: UNNEST ... WITH ORDINALITY (AND POSSIBLY OTHER STUFF)

On Fri, Nov 19, 2010 at 11:40:05AM +0900, Itagaki Takahiro wrote:
On Fri, Nov 19, 2010 at 08:33, David Fetter <david(at)fetter(dot)org<mailto:david(at)fetter(dot)org>> wrote:
In order to get WITH ORDINALITY, would it be better to change
gram.y to account for both WITH ORDINALITY and without, or just
for the WITH ORDINALITY case?

We probably need to change gram.y and make UNNEST to be
COL_NAME_KEYWORD. UNNEST (without ORDINALITY) will call the
existing unnest() function, and UNNEST() WITH ORDINALITY will call
unnest_with_ordinality().

Thanks for sketching that out :)

BTW, what will we return for arrays with 2 or more dimensions?

At the moment, per the SQL standard, UNNEST without the WITH
ORDINALITY clause flattens all dimensions.

SELECT * FROM UNNEST(ARRAY[[1,2],[3,4]]);
unnest
--------
1
2
3
4
(4 rows)

Unless we want to do something super wacky and contrary to the SQL
standard, UNNEST(array) WITH ORDINALITY should do the same.

There are no confusion in your two arguments version:
UNNEST(anyarray, number_of_dimensions_to_unnest)
but we will also support one argument version. Array indexes will
be composite numbers in the cases. The possible design would be just
return sequential serial numbers of the values -- the following two
queries return the same results:

- SELECT i, v FROM UNNEST($1) WITH ORDINALITY AS t(v, i)
- SELECT row_number() OVER () AS i, v FROM UNNEST($1) AS t(v)

Yes, that's what the standard says. Possible less-than-total
unrolling schemes include:

- Flatten specified number of initial dimensions into one list, e.g.
turn UNNEST(array_3d, 2) into SETOF(array_1d) with one column of
ORDINALITY

- Flatten similarly, but have an ORDINALITY column for each flattened
dimension.

- More exotic schemes, such as UNNEST(array_3d, [1,3]), with either of
the two methods above.

And of course the all-important:

- Other possibilities I haven't thought of :)

Cheers,
David.
--
David Fetter <david(at)fetter(dot)org<mailto:david(at)fetter(dot)org>> http://fetter.org/
Phone: +1 415 235 3778 AIM: dfetter666 Yahoo!: dfetter
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