Re: understanding handler::read_time and handler::scan_time

[Michael Widenius]

Hi!

>>>>> "Zardosht" == Zardosht Kasheff <zardosht@stripped> writes:

Zardosht> I realize this may be difficult to answer, but any examples on when
Zardosht> absolute values matter?

There is some constants in MySQL that for example assumes that
checking the WHERE condition for X numbers of rows takes as long as a
disk seek.  If you give very low absolute values for example for
scan() then it will affect the query plan used.

Think of like MySQL would calculate the cost as:

query_cost= min(scan_time + number_of_rows_scanned / time_to_process a row,
                read_time + number_of_rows / time_to_process a row)

The best way to get the right numbers is to estimate it in the number
of 20 milliseconds (a normal seek time) it takes to do one of the
operations.

Regards,
Monty
You can find MariaDB at http://www.askmonty.org