Re: Parameters don't work in FETCH NEXT clause?

On Tue, May 17, 2016 at 12:15 PM, Shay Rojansky <roji(at)roji(dot)org> wrote:

> Apologies, as usual I didn't read the docs carefully enough.
>
> On Tue, May 17, 2016 at 7:13 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>> Shay Rojansky <roji(at)roji(dot)org> writes:
>> > A user of mine just raised a strange issue... While it is possible to
>> use a
>> > parameter in a LIMIT clause, PostgreSQL does not seem to allow using
>> one in
>> > a FETCH NEXT clause. In other words, while the following works:
>> > SELECT 1 LIMIT $1;
>> > The following generates a syntax error:
>> > SELECT 1 FETCH NEXT $1 ROWS ONLY;
>> > Since LIMIT and FETCH NEXT are supposed to be equivalent this behavior
>> is
>> > odd.
>>
>> Per the SELECT reference page:
>>
>> SQL:2008 introduced a different syntax to achieve the same result,
>> which PostgreSQL also supports. It is:
>>
>> OFFSET start { ROW | ROWS }
>> FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
>>
>> In this syntax, to write anything except a simple integer constant for
>> start or count, you must write parentheses around it.
>>
>> The comments about this in gram.y are informative:
>>
>> * Allowing full expressions without parentheses causes various parsing
>> * problems with the trailing ROW/ROWS key words. SQL only calls for
>> * constants, so we allow the rest only with parentheses. If omitted,
>> * default to 1.
>>
>
​Would something like this be valid?

OFFSET { start_literal | ( start_expression ) } { ROW | ROWS }
FETCH { FIRST | NEXT} [ count_literal | ( count_expression ) ] { ROW | ROWS
} ONLY

Leaving the mandatory parentheses detail to the description, while
adequate, seems insufficient - especially when a normal LIMIT expression is
not so restricted.

​And don't you think the section header would be more accurately named:

Limit, Offset & Fetch Clauses

​The nuance regarding "different standard syntax" is unknown to the reader
who first looks at the syntax and sees three different lines, one for each
clause, and then scrolls down looking at headers until they find the
section for the clause they are interested in. That FETCH is an alias for
LIMIT ​is not something that I immediately understood - though to be honest
I don't think I comprehended the presence of FETCH on a SELECT query at all
and thought it only pertained to cursors....

David J.