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Date: Wed, 3 Jul 2024 11:00:17 +0200
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Subject: Re: COALESCE documentation
To: =?UTF-8?B?TmF2csOhdGlsLCBPbmTFmWVq?= <onavratil@monetplus.cz>,
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From: Peter Eisentraut <peter@eisentraut.org>
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On 02.07.24 12:45, Navrátil, Ondřej wrote:
> Hello,
> 
> as per documentation 
> <https://www.postgresql.org/docs/current/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL>
>  > The |COALESCE| function returns the first of its arguments that is 
> not null. Null is returned only if all arguments are null.
> 
> This is not exactly true. In fact:
> The |COALESCE| function returns the first of its arguments that *is 
> distinct* *from *null. Null is returned only if all arguments *are not 
> distinct from* null.
> 
> See my stack overflow question here 
> <https://stackoverflow.com/questions/78691097/postgres-null-on-composite-types>.
> 
> Long story short
> 
> |select coalesce((null, null), (10, 20)) as magic; |
> 
> returns
> 
> |magic ------- (,) (1 row)|
> 
> However, this is true:
> 
> |select (null, null) is null;|

I think this is actually a bug in the implementation, not in the 
documentation.  That is, the implementation should behave like the 
documentation suggests.