Обсуждение: BUG #9552: IndexOnlyScan startup_cost>0, why not equal 0?it's a bug?

The following bug has been logged on the website:

Bug reference:      9552
Logged by:          digoal.zhou
Email address:      digoal@126.com
PostgreSQL version: 9.3.3
Operating system:   CentOS 6.4 x64
Description:

I belive index only scan can return tuple direct, it's not need to scan
heappage, why it's startup_cost equal to index scan?
I'ts a bug?

The TEST below:
digoal=# create table t11(id int primary key, info text);
CREATE TABLE
digoal=# insert into t11 select generate_series(1,100000),'test';
INSERT 0 100000
digoal=# explain select * from t11 where id=1;
                             QUERY PLAN
--------------------------------------------------------------------
 Index Scan using t11_pkey on t11  (cost=0.29..4.31 rows=1 width=9)
   Index Cond: (id = 1)
(2 rows)

digoal=# explain select id from t11 where id=1;
                               QUERY PLAN
-------------------------------------------------------------------------
 Index Only Scan using t11_pkey on t11  (cost=0.29..4.31 rows=1 width=4)
   Index Cond: (id = 1)
(2 rows)

Hello,

> I belive index only scan can return tuple direct, it's not need to scan
> heappage, why it's startup_cost equal to index scan?
> I'ts a bug?

No. It is a result of reasonable calculus.

>  Index Scan using t11_pkey on t11  (cost=0.29..4.31 rows=1 width=9)

Index scan runs through several disk (but usually on cache)
blocks during descending down to the leaf node in the index tree
and does comparisons for certain amount of index entries(tuples)
in each index node with regard to btree. The figure made from the
expected number of the comparisons multiplied by
cpu_operator_cost. Each access methods (index types) has their
own function to do that.

=# select amname, amcostestimate from pg_am where amname = 'btree';
 amname | amcostestimate
--------+----------------
 btree  | btcostestimate

The function 'btcostestimate' in selfuncs.c calculates it for
btree index. You can see the details there if you wish.

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center

digoal@126.com writes:
> I belive index only scan can return tuple direct, it's not need to scan
> heappage, why it's startup_cost equal to index scan?
> I'ts a bug?

No.  Your test case fails to vacuum t11, so although the planner selects
an index-only scan, it's still predicting that all the tuples will require
heap visits to confirm visibility.

The startup cost would not change in any case, since that's about index
descent costs not heap visit costs.

            regards, tom lane

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