Re: BUG #14059: BUG? function in select clause called more times when use offset

At 2016-04-02 22:49:26, "Tom Lane" <tgl@sss.pgh.pa.us> wrote:
>digoal@126.com writes:
>> postgres=# create or replace function f() returns void as $$
>> declare
>> begin
>>   raise notice 'called';
>> end;
>> $$ language plpgsql strict volatile;
>> CREATE FUNCTION
>
>> postgres=# select f(),* from ( values(1),(2),(3),(4),(5),(6) ) t(id) offset
>> 3 limit 2;
>> NOTICE:  called
>> NOTICE:  called
>> NOTICE:  called
>> NOTICE:  called
>> NOTICE:  called
>>  f | id 
>> ---+----
>>    |  4
>>    |  5
>> (2 rows)
>
>> offset skip 3 tuple, but function f() called with 5 times, can we tuning
>> this .
>> or this is a bug?
>
>No, it's not a bug.  OFFSET only results in the skipped tuples not being
>delivered to the client; it does not cause them not to be computed.
>
>You could probably do something with a two-level select with the OFFSET
>in the sub-select and the volatile function in the top level.
>
>			regards, tom lane