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* PostgreSQL 9.5 operator precedence
@ 2016-09-20 06:02 =?koi8-r?B?4c7E0sXKIOHXwcvJzc/X?= <aquarius1993@rambler.ru>
0 siblings, 1 reply; 3+ messages in thread
From: Андрей Авакимов @ 2016-09-20 06:02 UTC (permalink / raw)
To: pgsql-sql
Hello,
I've got a question about operator precedence is version 9.5
Versions lower than 9.5 worked well with constructions like
select a is null = b is null
(it calculates if a is null, then if b is null and then compare two booleans)
But now everything is changed, and official 9.5 documentations says that
IS operator has lower precedence than = operator
The thing I don't understand is the error message that I receive:
select 1 is null = 2 is null;
-----------------------------
ERROR: operator does not exist: boolean = integer
LINE 1: select 1 is null = 2 is null
^
HINT: No operator matches the given name and argument type(s). You might need to
add explicit type casts.
For me it means that postgres calculated value in the left side of the =
operator, then it decided not to calculate right side and compare boolean and
integer.
Why did it not compare null and 2 firstly?
Maybe the truth is IS operator and = operator has the same precedence, and then
calculated from left to right?
Maybe I miss something?
Best Regards,
Andrew
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: PostgreSQL 9.5 operator precedence
@ 2016-09-20 14:10 David G. Johnston <david.g.johnston@gmail.com>
parent: =?koi8-r?B?4c7E0sXKIOHXwcvJzc/X?= <aquarius1993@rambler.ru>
0 siblings, 1 reply; 3+ messages in thread
From: David G. Johnston @ 2016-09-20 14:10 UTC (permalink / raw)
To: Андрей Авакимов <aquarius1993@rambler.ru>; +Cc: pgsql-sql
On Mon, Sep 19, 2016 at 11:02 PM, Андрей Авакимов <aquarius1993@rambler.ru>
wrote:
> Hello,
> I've got a question about operator precedence is version 9.5
>
> Versions lower than 9.5 worked well with constructions like
> select a is null = b is null
> (it calculates if a is null, then if b is null and then compare two
> booleans)
>
> But now everything is changed, and official 9.5 documentations says that
> IS operator has lower precedence than = operator
>
> The thing I don't understand is the error message that I receive:
>
> select 1 is null = 2 is null;
> -----------------------------
> ERROR: operator does not exist: boolean = integer
> LINE 1: select 1 is null = 2 is null
> ^
> HINT: No operator matches the given name and argument type(s). You
> might need to add explicit type casts.
>
Your query reads:
SELECT ( ( 1 IS (NULL = 2) ) IS NULL
)
So the error is in:
1 IS (NULL = 2)
which indeed is integer = boolean
David J.
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: PostgreSQL 9.5 operator precedence
@ 2016-09-20 14:47 Tom Lane <tgl@sss.pgh.pa.us>
parent: David G. Johnston <david.g.johnston@gmail.com>
0 siblings, 0 replies; 3+ messages in thread
From: Tom Lane @ 2016-09-20 14:47 UTC (permalink / raw)
To: David G. Johnston <david.g.johnston@gmail.com>; +Cc: Андрей Авакимов <aquarius1993@rambler.ru>; pgsql-sql
"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Mon, Sep 19, 2016 at 11:02 PM, Андрей Авакимов <aquarius1993@rambler.ru>
>> The thing I don't understand is the error message that I receive:
>> select 1 is null = 2 is null;
>> -----------------------------
>> ERROR: operator does not exist: boolean = integer
>> LINE 1: select 1 is null = 2 is null
> Your query reads:
> SELECT ( ( 1 IS (NULL = 2) ) IS NULL
No, certainly not that --- IS isn't some sort of standalone operator,
rather IS NULL is an indivisible combination of tokens representing a
postfix operator. The query's really getting parsed like this:
select ((1 is null) = 2) is null;
whereas the pre-9.5 interpretation was
select (1 is null) = (2 is null);
If you add those parentheses explicitly then your query will work fine
in either version.
regards, tom lane
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