Re: Transforming IN (...) to ORs, volatility

On 05.04.2011 18:42, Heikki Linnakangas wrote:
> On 05.04.2011 13:19, Marti Raudsepp wrote:
>> On Fri, Apr 1, 2011 at 14:24, Heikki Linnakangas
>> <heikki.linnakangas@enterprisedb.com> wrote:
>>> We sometimes transform IN-clauses to a list of ORs:
>>>
>>> postgres=# explain SELECT * FROM foo WHERE a IN (b, c);
>>> QUERY PLAN
>>> Seq Scan on foo (cost=0.00..39.10 rows=19 width=12)
>>> Filter: ((a = b) OR (a = c))
>>>
>>> But what if you replace "a" with a volatile function? It doesn't seem
>>> legal
>>> to do that transformation in that case, but we do it:
>>>
>>> postgres=# explain SELECT * FROM foo WHERE (random()*2)::integer IN
>>> (b, c);
>>> QUERY PLAN
>>>
>>> Seq Scan on foo (cost=0.00..68.20 rows=19 width=12)
>>> Filter: ((((random() * 2::double precision))::integer = b) OR
>>> (((random()
>>> * 2::double precision))::integer = c))
>>
>> Is there a similar problem with the BETWEEN clause transformation into
>> AND expressions?
>>
>> marti=> explain verbose select random() between 0.25 and 0.75;
>> Result (cost=0.00..0.02 rows=1 width=0)
>> Output: ((random()>= 0.25::double precision) AND (random()<=
>> 0.75::double precision))
>
> Yes, good point.

Hmm, the SQL specification explicitly says that

X BETWEEN Y AND Z

is equal to

X >= Y AND X <= Z

It doesn't say anything about side-effects of X. Seems like an oversight 
in the specification. I would not expect X to be evaluated twice, and I 
think we should change BETWEEN to not do that.


Does anyone object to making BETWEEN and IN more strict about the data 
types? At the moment, you can do this:

postgres=# SELECT '1234' BETWEEN '10001'::text AND 10002::int4; ?column?
---------- t
(1 row)

I'm thinking that it should throw an error. Same with IN, if the values 
in the IN-list can't be coerced to a common type. That will probably 
simplify the code a lot, and is what the SQL standard assumes anyway AFAICS.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com