Re: empty arrays

Caution!

In 7.2.x your statement is interpreted by the parser to be a single element
with an empty string which is converted to a zero. If you do this instead:

create table test_table ( test_column integer[], another_column integer );
CREATE
steve=# insert into test_table (another_column) values (1);
INSERT 7140977 1
steve=# select * from test_table;
test_column | another_column
-------------+----------------
| 1
(1 row)

You will see that there is no item in the array which you can confirm with:
select array_dims(test_column) from test_table;
array_dims
------------

(1 row)
(Which seemingly oddly shows nothing instead of 0 since there is not yet
really any array to have dimensions. Per the docs, an array can be null but
it cannot contain SQL null elements. What the docs don't say is that
requesting an element beyond either end of an array does return a SQL null.)

Here's the gotcha: when you upgrade to 7.3.x your statement will generate an
error since an empty string is no longer arbitrarily assumed to be a zero
(either in integer fields or arrays) so if you attempt the same you will get:

create table test_table ( test_column integer[], another_column integer );
CREATE TABLE
tati=> insert into test_table (test_column) values ( '{ }' );
ERROR: pg_atoi: zero-length string

If you want to insert a null array you are best off saying so:
insert into test_table (test_column) values (null);
INSERT 17053 1
tati=> select * from test_table;
test_column | another_column
-------------+----------------
|
(1 row)

Cheers,
Steve

On Monday 30 December 2002 9:40 am, floyds(at)4peakstech(dot)com wrote:
> using: psql (PostgreSQL) 7.2.1
>
> why does an empty array return an array of length 1 rather than array of
> length 0? one would think that the results below would have returned { }
> instead of {0}.
>
> simple test using psql:
>
> # create table test_table ( test_column integer[] );
> CREATE
> # insert into test_table (test_column) values ( '{ }' );
> INSERT 43475 1
> # select * from test_table;
>
> test_column
> ------------
> {0}
> (1 row)
>
>
> i want to be able to store a zero-length array field in the database. how
> do i specify this with sql?
>
> Regards,
>
> Floyd Shackelford
> 4 Peaks Technology Group, Inc.
> VOICE: 334.735.9428
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