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<st...@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. >> > > -- > st...@hibernate.org > http://hibernate.org > > -- > st...@hibernate.org > http://hibernate.org _______________________________________________ hibernate-dev mailing list hibernate-dev@lists.jboss.org https://lists.jboss.org/mailman/listinfo/hibernate-dev
