On Sun, Apr 6, 2008 at 9:57 PM, Eric Bergen <eric@stripped> 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@stripped
wultsch (aim)
