Re: Innodb vs myisam

["Krishna Chandra Prajapati" <prajapatikc@xxxxxxxxx>]

Thanks a lot

On Mon, Apr 7, 2008 at 10:59 AM, Rob Wultsch <wultsch@xxxxxxxxx> wrote:

> On Sun, Apr 6, 2008 at 9:57 PM, Eric Bergen <eric@xxxxxxxxxxxxxxxxx>
> wrote:
> > I don't see what the issue is. As Jay said the row counts in explain
> >  outputs are estimates. When running an explain query MySQL asks the
> >  storage engine how many rows it thinks are between a set of values for
> >  an index. Different storage engines use different methods to calculate
> >  row count. Both innodb and myisam estimate the row count  based on
> >  statistics they keep on the distribution of keys in an index. MyISAM
> >  is more accurate than innodb with it's row count because of how it
> >  keeps statistics. Analyze table on a myisam table will count the
> >  number of unique values in an index
> >  (myisam/mi_check:update_key_parts). Innodb samples the key
> >  distribution in 8 different pages per index and does some calculations
> >  based on the tree structure of those pages (details
> >  innobase/btr/btr0cur.c:btr_estimate_number_of_different_key_vals).
>
> Thank you! I spent a bit trying to find details about how row counts
> were computed, but with no luck (I probably did not know how to know
> how to look...). Could the alternative I had proposed be accurate at
> all?
>
> --
> Rob Wultsch
> wultsch@xxxxxxxxx
> wultsch (aim)
>



-- 
Krishna Chandra Prajapati