Re: Difference between CAST(v AS t) and v::t

Lele Gaifax <lele(at)metapensiero(dot)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