Обсуждение: Empty arrays vs. NULLs, 9.1 & 8.3
Thanks Steve. FWIW I looked at the 9.0 and 9.1 release notes, and didn't find much on arrays in them.
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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:
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.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!
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
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