[YSQL] Index scan cost does not account for disk fetch cost correctly

[gauravk-in]

Jira Link: DB-10792

Description

The following query is from cost-validation benchmark in TAQO.

create table t100000 (c1 int, c2 int not null, c3 int, c4 int, c5 int, c6 int, v char(1024), primary key (c1 asc));
create index t100000_c2_c4 on t100000 (c2 asc) include (c4);
create index t100000_c2_c4v on t100000 (c2 asc) include (c4, v);

insert into t100000
  select i, i2,
         nullif((i3+4)/5, (100000+4)/5),
         nullif((i4+9)/10, (100000+9)/10),
         i5,
         nullif((i6+99)/100, (100000+99)/100),
         lpad(sha512(i5::text::bytea)||sha512(i::text::bytea)::text, 1024, '-')
    from (
      select i,
          row_number() over (order by random()) i2,
          row_number() over (order by random() + 1) i3,
          row_number() over (order by random() + 2) i4,
          row_number() over (order by random() + 3) i5,
          row_number() over (order by random() + 4) i6
        from generate_series(1, 100000) i
    ) v order by 1;

ANALYZE t100000;

-- The following queries have the same cost, even though the t100000_c2_c4v has included column v which is 8KB wide.

/*+ IndexOnlyScan(t100000_c2_c4) */ explain SELECT avg(c4) FROM t100000;
/*+ IndexOnlyScan(t100000_c2_c4v) */ explain SELECT avg(c4) FROM t100000;

Issue Type

kind/bug

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