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

HI,
Thanks, but IndexOnlyScan's startup_cost not contain VM scan cost?

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=# select reltuples from pg_class where relname='t11_pkey';
reltuples
-----------
100000
(1 row)
digoal=# show cpu_operator_cost;
cpu_operator_cost
-------------------
0.0025
(1 row)
digoal=# select (log(100000)/log(2.0))*0.0025;
?column?
-------------------
0.041524101186092
(1 row)
digoal=# create extension pageinspect;
CREATE EXTENSION
digoal=# SELECT * FROM bt_metap('t11_pkey');
magic | version | root | level | fastroot | fastlevel
--------+---------+------+-------+----------+-----------
340322 | 2 | 3 | 1 | 3 | 1
(1 row)
digoal=# select (1+1)*50*0.0025;
?column?
----------
0.2500
(1 row)
digoal=# select 0.041524101186092+0.2500;
?column?
-------------------
0.291524101186092
(1 row)

##########################
We can see index scan and index only scan startup_cost is same below, indexonlyscan not contain vm scan cost?
And index only scan real slower than index scan.

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)

digoal=# explain (analyze,verbose,costs,buffers,timing) select id from t11 where id<10000;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using t11_pkey on public.t11 (cost=0.29..204.20 rows=9995 width=4) (actual time=0.088..3.623 rows=9999 loops=1)
Output: id
Index Cond: (t11.id < 10000)
Heap Fetches: 9999
Buffers: shared hit=22
Total runtime: 4.748 ms
(6 rows)

digoal=# vacuum ANALYZE t11;
VACUUM
digoal=# explain (analyze,verbose,costs,buffers,timing) select id from t11 where id<10000;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using t11_pkey on public.t11 (cost=0.29..191.64 rows=10020 width=4) (actual time=0.088..2.597 rows=9999 loops=1)
Output: id
Index Cond: (t11.id < 10000)
Heap Fetches: 0
Buffers: shared hit=9
Total runtime: 3.634 ms
(6 rows)

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From: Tom Lane
Date: 2014-03-13 21:56
To: digoal
CC: pgsql-bugs
Subject: Re: [BUGS] BUG #9552: IndexOnlyScan startup_cost>0, why not equal 0?it's a bug?
digoal(at)126(dot)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