Hi, Anshu!

On Jun 11, Anshu Avinash wrote:
> Hi serg,
>

> I have attached the updated diff. I'm still unable to observe the effect
> introduced by the cost factors. Maybe I just need to study the usage of
> scan_time() and read_time() in greater detail.

Ok, see one suggestion below. If that won't help - push your changes to
github, I'll try myself.

> diff --git a/mysql-test/r/costmodel.result b/mysql-test/r/costmodel.result
> new file mode 100644
> index 0000000..6668758
> --- /dev/null
> +++ b/mysql-test/r/costmodel.result
> @@ -0,0 +1,20 @@

> +DROP TABLE IF EXISTS t1;

> +CREATE TABLE t1 (a int auto_increment primary key, b int);
> +INSERT INTO t1(b) values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12),
> +(13),(14),(15),(16),(17),(18),(19),(20),(21),(22),(23),(24),(25);
> +EXPLAIN
> +SELECT * FROM t1
> +WHERE a > 21;
> +id   select_type     table   type    possible_keys   key     key_len ref     rows    Extra
> +1    SIMPLE  t1      range   PRIMARY PRIMARY 4       NULL    5       Using index condition
> +use mysql;
> +UPDATE optimizer_cost_factors
> +SET const_value=2.0
> +WHERE const_name='SCAN_TIME_FACTOR';
> +use test;
> +EXPLAIN
> +SELECT * FROM t1
> +WHERE a > 21;
> +id   select_type     table   type    possible_keys   key     key_len ref     rows    Extra
> +1    SIMPLE  t1      range   PRIMARY PRIMARY 4       NULL    5       Using index condition
> +DROP TABLE t1;

See? Your first EXPLAIN already shows type=range. And then you increase
the cost of the table scan. Of course, nothing can happen anymore.

Try to make the first explain to show type=ALL (which means table scan),
then increase the cost of the table scan and see how optimizer will
switch to type=range.

Regards,
Sergei