Thanks a lot On Mon, Apr 7, 2008 at 10:59 AM, Rob Wultsch <[EMAIL PROTECTED]> wrote:
> On Sun, Apr 6, 2008 at 9:57 PM, Eric Bergen <[EMAIL PROTECTED]> > 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 > [EMAIL PROTECTED] > wultsch (aim) > -- Krishna Chandra Prajapati
