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