[hibernate-dev] Memory consumption

[Steve Ebersole]

Besides, how is the plan any different for:

((id = null) OR (id = null) OR (id = null) OR (id = 4) OR (id = 5))


On 05/17/2012 10:40 AM, Steve Ebersole wrote:
> Its an index scan, totally trivial (unless their index hashing algos 
> are awful as well).
>
>
> On Thu 17 May 2012 08:49:06 AM CDT, Guillaume Smet wrote:
>> On Thu, May 17, 2012 at 3:02 PM, Steve Ebersole<steve at hibernate.org>  
>> wrote:
>>> But just because you have 100 values in no way indicates how many 
>>> rows will
>>> be returned.  And I personally know of no optimizers that make such an
>>> assumption; its a totally worthless assumption.
>>
>> That's why I put "simplified" in front of it.
>>
>> The number of clauses is taken into account to calculate the
>> estimation of the number of rows returned and depending on the plan
>> chosen to get the results, this might lead to results you don't expect
>> and don't want.
>>
>> See the below example on PostgreSQL (which is far from having the
>> worst optimizer/planner of the market):
>> sitra=# explain select id from objettouristique where id=4 or id=4 or
>> id=4 or id=4 or id=5;
>>                                          QUERY PLAN
>> ------------------------------------------------------------------------------------------ 
>>
>>   Bitmap Heap Scan on objettouristique  (cost=1.80..2.41 rows=5 width=8)
>>     Recheck Cond: ((id = 4) OR (id = 4) OR (id = 4) OR (id = 4) OR 
>> (id = 5))
>>     ->   BitmapOr  (cost=1.80..1.80 rows=5 width=0)
>>           ->   Bitmap Index Scan on objettouristique_pkey
>> (cost=0.00..0.36 rows=1 width=0)
>>                 Index Cond: (id = 4)
>>           ->   Bitmap Index Scan on objettouristique_pkey
>> (cost=0.00..0.36 rows=1 width=0)
>>                 Index Cond: (id = 4)
>>           ->   Bitmap Index Scan on objettouristique_pkey
>> (cost=0.00..0.36 rows=1 width=0)
>>                 Index Cond: (id = 4)
>>           ->   Bitmap Index Scan on objettouristique_pkey
>> (cost=0.00..0.36 rows=1 width=0)
>>                 Index Cond: (id = 4)
>>           ->   Bitmap Index Scan on objettouristique_pkey
>> (cost=0.00..0.36 rows=1 width=0)
>>                 Index Cond: (id = 5)
>>
>> It sure looks stupid but the opinion of the PostgreSQL hackers is that
>> it's stupid to add several times the same clause. The cost of checking
>> all the conditions can be high and is wasted most of the time.
>>
>
> -- 
> steve at hibernate.org
> http://hibernate.org
>
> -- 
> steve at hibernate.org
> http://hibernate.org