Empty arrays vs. NULLs, 9.1 & 8.3

On 06/14/12 12:47 PM, Ken Tanzer wrote:

p.s., On a side note, unless I've overlooked them before, the "this
page in other versions..." links in the doc pages seem to be new, and
are immensely helpful. Especially because Google searches often
return results to the older versions.

indeed, thats exactly why it was done.

--
john r pierce N 37, W 122
santa cruz ca mid-left coast

On 06/14/2012 12:47 PM, Ken Tanzer wrote:

Hi. I had this piece of SQL, which ran fine on my 9.1 installation:

INSERT INTO foo
SELECT ...,
CASE WHEN NOT allow_output_screen THEN array['O_SCREEN'] ELSE
array[]::varchar[] END
|| CASE WHEN NOT allow_output_spreadsheet THEN array['O_TEMPLATE']
ELSE array[]::varchar[] END,
...;

However, this failed miserably on someone else's 8.3:

ERROR: syntax error at or near "]"
LINE 1: ...w_output_screen THEN array['O_SCREEN'] ELSE array[]::varchar

^
The 9.1 documentation
(http://www.postgresql.org/docs/9.1/static/sql-expressions.html)
states you can construct an empty array with my syntax (that's how I
got it originally), but there is no mention of empty arrays in the
corresponding 8.3 page.

In 8.3, I can SELECT NULL::varchar[], which seems to behave the same
in my query. The two don't seem to be exactly the same. I'm a little
confused, however, as to the finer points or conceptual differences
between them, and also what the differences might be between 8.3 and
9.1.

Sticking within 9.1, I ran this:

=>CREATE TEMPORARY TABLE array_fun( f1 varchar[] );
=>INSERT INTO array_fun VALUES (array[]::varchar[]),(NULL::varchar[]);

CREATE TABLE
INSERT 0 2

=> SELECT array_dims(f1) AS dims, f1 IS NULL as is_null, f1 ||
array['Item 2']::varchar[] AS concats,f1 FROM array_fun;

dims | is_null | concats | f1
------+---------+------------+----
| f | {"Item 2"} | {}
| t | {"Item 2"} |
(2 rows)

If anyone can shed some light on this, and also how to construct an
empty array in 8.3, it would be great. Thanks!

Array handling in general has undergone many changes from 8.3 to 9.1 and
more if you go back from 8.3. Check the release notes for each major
release for the specifics and logic. Some of the changes such as how to
handle string_to_array('') (my fault for bringing it up originally)
required discussions that spanned a couple major versions.

To create an empty array in 8.3 you can just use '{}' and note that an
empty array is *not* null while NULL::varchar[] *is*.

Cheers,
Steve

Thanks Steve. FWIW I looked at the 9.0 and 9.1 release notes, and didn't
find much on arrays in them.

I do have one follow-up curiosity question, though. Why does
array_dims(array[]::varchar[]) return NULL instead of 0? I would expect
NULL for a NULL array, but not an empty one. (And the same for
array_[upper,lower,length] functions as well.

There doesn't seem to be much coverage of NULLs in the array documentation,
so in the making-work-for-other-people department, I'd suggest that either
weaving it in or including a small separate section on the topic might be
helpful.

Cheers,
Ken

On Thu, Jun 14, 2012 at 3:15 PM, Steve Crawford <
scrawford@pinpointresearch.com> wrote:

On 06/14/2012 12:47 PM, Ken Tanzer wrote:

Hi. I had this piece of SQL, which ran fine on my 9.1 installation:

INSERT INTO foo
SELECT ...,
CASE WHEN NOT allow_output_screen THEN array['O_SCREEN'] ELSE
array[]::varchar[] END
|| CASE WHEN NOT allow_output_spreadsheet THEN array['O_TEMPLATE']
ELSE array[]::varchar[] END,
...;

However, this failed miserably on someone else's 8.3:

ERROR: syntax error at or near "]"
LINE 1: ...w_output_screen THEN array['O_SCREEN'] ELSE array[]::varchar

^
The 9.1 documentation
(http://www.postgresql.org/**docs/9.1/static/sql-**expressions.html&lt;http://www.postgresql.org/docs/9.1/static/sql-expressions.html&gt;
)
states you can construct an empty array with my syntax (that's how I
got it originally), but there is no mention of empty arrays in the
corresponding 8.3 page.

In 8.3, I can SELECT NULL::varchar[], which seems to behave the same
in my query. The two don't seem to be exactly the same. I'm a little
confused, however, as to the finer points or conceptual differences
between them, and also what the differences might be between 8.3 and
9.1.

Sticking within 9.1, I ran this:

=>CREATE TEMPORARY TABLE array_fun( f1 varchar[] );
=>INSERT INTO array_fun VALUES (array[]::varchar[]),(NULL::**varchar[]);

CREATE TABLE
INSERT 0 2

=> SELECT array_dims(f1) AS dims, f1 IS NULL as is_null, f1 ||
array['Item 2']::varchar[] AS concats,f1 FROM array_fun;

dims | is_null | concats | f1
------+---------+------------+**----
| f | {"Item 2"} | {}
| t | {"Item 2"} |
(2 rows)

If anyone can shed some light on this, and also how to construct an
empty array in 8.3, it would be great. Thanks!

Array handling in general has undergone many changes from 8.3 to 9.1 and
more if you go back from 8.3. Check the release notes for each major
release for the specifics and logic. Some of the changes such as how to
handle string_to_array('') (my fault for bringing it up originally)
required discussions that spanned a couple major versions.

To create an empty array in 8.3 you can just use '{}' and note that an
empty array is *not* null while NULL::varchar[] *is*.

Cheers,
Steve

--
AGENCY Software
A data system that puts you in control
*http://agency-software.org/*
ken.tanzer@agency-software.org
(253) 245-3801

On 06/14/2012 03:54 PM, Ken Tanzer wrote:

Thanks Steve. FWIW I looked at the 9.0 and 9.1 release notes, and
didn't find much on arrays in them.

The notes are terse (a 1-2 line comment encapsulates the results of
hundreds of messages covering a couple year) but there are many
references to updates in how arrays are handled.

I do have one follow-up curiosity question, though. Why does
array_dims(array[]::varchar[]) return NULL instead of 0? I would
expect NULL for a NULL array, but not an empty one. (And the same for
array_[upper,lower,length] functions as well.

I can't answer, specifically. But it makes a certain amount of sense. An
empty array has no elements so where does it "begin" or "end" -
especially since PostgreSQL array indexes don't need to start at 1 and
the array could be multi-dimensional. Unknown, aka null, seems the best
response.

Cheers,
Steve

Ken Tanzer <ken.tanzer@gmail.com> writes:

Thanks Steve. FWIW I looked at the 9.0 and 9.1 release notes, and didn't
find much on arrays in them.

This particular issue was fixed in 8.4.

I do have one follow-up curiosity question, though. Why does
array_dims(array[]::varchar[]) return NULL instead of 0? I would expect
NULL for a NULL array, but not an empty one. (And the same for
array_[upper,lower,length] functions as well.

Yeah, there are still a lot of inconsistencies there :-(. Eventually
I'd like to see somebody go through all the array operations and make
a proposal for consistent handling of empty arrays. I think it would
be better if we changed all those things at once, rather than causing
piecemeal compatibility hits.

regards, tom lane