Olivier Doucet <webmas...@ajeux.com> was heard to say:

> Hello everyone,
> I'm following a quite old topic about libdbi speed issues.
> I was able to track the cause of these issues : The major problem is
> how libdbi goes from one row to another.
> RRDTool (the tool that used libdbi and that I was inspecting) is using
> dbi_result_next_row() function (as stated in libdbi documentation
> btw).
> This function moves from one row to another with function
> dbi_result_seek_row(), incrementing currentRow index each time. This
> gives a call to dbd_mysql.c::dbd_goto_row() that uses
> mysql_data_seek() each time...
> That's why for a query result of 34k rows (yes it happens. No it is
> not a problem in the query itself), we have tens of thousands of call
> to this function (which is very low), and this is definitely not
> needed, because as we use fetch_row(), we automatically move from one
> row to another. Seeking is just a useless task (as internal driver
> does not know where we are, and needs to start from row 0 and seek to
> the given row - where we already were).
> I'm absolutely not a libdbi user, and I don't know what could be done
> outside libdbi to not use dbi_result_next_row() and use directly
> RESULT->onn->driver->functions->fetch_row() directly. Is it possible ?
> And/or patching dbi_result.c :
> just check RESULT->currowidx near line 102 before calling doing
> goto_row() function and call it only if we are not on the good row. Am
> I right ?


your analysis is pretty much correct. If you look at the comments in  
dbd_mysql.c::dbd_goto_row(), the original author of the mysql driver  
was well aware of the limitations of his implementation. The reason is  
that other database APIs, e.g. PostgreSQL, allow to fetch rows from a  
result set by index, whereas the MySQL API assumes that you step  
through the rows sequentially. The original design of libdbi appears  
to somewhat favor PostgreSQL in this respect.

Anyway, without having thought about the issue in too much detail, one  
possible solution comes to mind. We could modify the driver function  
dbd_goto_row() by passing both the wanted row index rowidx and the  
current row index currowidx(which libdbi keeps track of anyway). This  
would allow drivers to decide whether they have to actually seek the  
position. pgsql doesn't have to anyway, and mysql doesn't have to if  
rowidx = currowidx+1. This API change would not mandate changes to  
existing drivers as they may ignore the additional parameter and keep  
working as before, but it may offer options to speed up queries in  
some drivers.


Markus Hoenicka
AQ score 38

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