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Date: Thu, 09 Jul 2015 02:31:10 +0200
From: Michael Herold <quabla@hemio.de>
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Subject: ARRAY[1] || NULL <> array_append(ARRAY[1], NULL)
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A few minutes ago I got bitten by the following PostgreSQL (9.4) behavior

SELECT ARRAY[1] || NULL, array_append(ARRAY[1], NULL);
  ?column? | array_append
----------+--------------
  {1}      | {1,NULL}

I expected that array_append and || are equivalent in this case but 
obviously they are not. Sure, this is not too surprising since "||" has 
to guess which operation is appropriate. However, I would have highly 
appreciated ARRAY[1,2] || NULL as an example in [Table 9-45]. Combined 
with the example of NULL || ARRAY[1,2] the underlying principle becomes 
clear to me.

Strings behave different, but maybe this is also a potential pitfall:

SELECT 'abc' || NULL, concat('abc', NULL);
  ?column? | concat
----------+--------
  (NULL)   | abc

Best,
Michael

[Table 9-45]: 
<http://www.postgresql.org/docs/9.4/static/functions-array.html#ARRAY-OPERATORS-TABLE>


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