Обсуждение: [GENERAL] Difference between CAST(v AS t) and v::t

Hi all,

while writing test cases for my SQL pretty printer tool[1], I found what seems
a discrepancy in the "Type Casts" documentation[2]: it states that the two
syntaxes are equivalent, but while
 CREATE TABLE contracts (   ...   company_id uuid NOT NULL,   validity daterange NOT NULL,      EXCLUDE USING gist
(CAST(company_idAS text) WITH =, validity WITH &&) )
 

works, the following
 CREATE TABLE contracts (   ...   company_id uuid NOT NULL,   validity daterange NOT NULL,      EXCLUDE USING gist
(company_id::textWITH =, validity WITH &&) )
 

is rejected with the message "syntax error at or near "::"".

Am I misreading the documentation, or is it missing some special case?

Thanks in advance for any explanation,
ciao, lele.

[1] https://github.com/lelit/pg_query
[2] https://www.postgresql.org/docs/10/static/sql-expressions.html#SQL-SYNTAX-TYPE-CASTS
-- 
nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri
real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia.
lele@metapensiero.it  |                 -- Fortunato Depero, 1929.



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Lele Gaifax <lele@metapensiero.it> writes:
> while writing test cases for my SQL pretty printer tool[1], I found what seems
> a discrepancy in the "Type Casts" documentation[2]: it states that the two
> syntaxes are equivalent, but while

They are functionally equivalent ...

>     EXCLUDE USING gist (CAST(company_id AS text) WITH =, validity WITH &&)
> works, the following
>     EXCLUDE USING gist (company_id::text WITH =, validity WITH &&)
> is rejected with the message "syntax error at or near "::"".

... but expression-index syntax has the restriction that you need
parentheses around an expression unless it is, or at least looks like,
a function call.  CAST() looks enough like a function call for this
purpose, v::t does not.

I think there is relevant documentation for this near CREATE INDEX;
it doesn't seem like the province of the typecast docs to explain
the weirdnesses of index syntax.
        regards, tom lane


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Tom Lane <tgl@sss.pgh.pa.us> writes:

> ... but expression-index syntax has the restriction that you need
> parentheses around an expression unless it is, or at least looks like,
> a function call.  CAST() looks enough like a function call for this
> purpose, v::t does not.
>
> I think there is relevant documentation for this near CREATE INDEX;
> it doesn't seem like the province of the typecast docs to explain
> the weirdnesses of index syntax.

Thank you Tom, I really missed the explanation in CREATE INDEX doc entry!

ciao, lele.
-- 
nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri
real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia.
lele@metapensiero.it  |                 -- Fortunato Depero, 1929.



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