> also something interesting
>
> if i do (wee yyy a completely new value)
> delete ... where xxx= yyy

Delete should do

a) purge old record(s) version(s) with the xxx = yyy (which you deleted and committed before).
This also removes entries for this old record(s) from all indices.
This is not cheap operation.
Note, this will not happen if there is concurrent live transaction interested in some of this old record(s) version(s).

b) create delete stub for currently alive record(s) with xxx = yyy.
This is relatively cheap, especially if data page with record have some free space. Else backversion will be placed at another data page and it adds some cost.

> Insert ... (xxx= yyy)

Insert should do

a) create new record somewhere
This is usually cheap

b) add entries for all indices
This is costly.

> commit => 650 ms (isql show read 2, write 297)

Commit will write all dirty pages on disk. Cost of this operation can be very different in dependance of

a) how many pages should be written

b) how far this pages on disk from each other

c) in which order this pages should be written : because of careful write strategy we have always consistent database image on disk but we can't fully optimize writes order.

For example, write 32 consecutive pages (1, 2, ..., 32) could have almost the same cost as write 2 pages (1, 32).
And write of the same 2 pages but in reverse order (32, 1) have much bigger cost.

Of course, if there is another activity on the same HDD whole picture could be very different.

> and i do again (exactly the same as previous)
> delete ... where xxx= yyy
> Insert ... (xxx= yyy)
> commit => 600 ms (isql show read 2, write 294)
>
> and i do again (exactly the same as previous)
> delete ... where xxx= yyy
> Insert ... (xxx= yyy)
> commit => 40 ms (isql show read 2, write 148)
>
> and it's always like this, at the 3th time (only) it's go fastly !
> why for the exact same intructions the commit need to write more in The 1rt and 2nd call than in the Third call ?

I'm afraid if i can't reproduce it by myself i can't said much about it. Probably, if you'll show separate stats for delete, insert and commit it will be more clear.

Regards,
Vlad