Changying Li schrieb:
why does mysql use group_id index ?
because in this case group_id would be faster than user_id
but in fact group_id is very slow (51.21 sec), user_id is (0.00 sec)
ok, at least MySQL does think so
I know, but I what I really want to know is how does mysql think so ?
because "rows" is smaller for this index, when deciding what index to use
it seems MySQL does not take into account what other/later steps needed (on
joined tables) to get the final result ...
how to let mysql choose user_id as an index ? what's the mean of 'rows' ?
valid rows after applying the WHERE to this index
the result is empty set, if what you said is true, then the rows must be
0 ?
no, not the final result, only for this index
read about EXPLAIN in the MySQL manual
http://dev.mysql.com/doc/refman/5.0/en/using-explain.html
I has read it , and it described like what you said, I don't really know
what is the mean of 'only for this index',
I tried 'select count(*) from photo where group_id=0 and album_id!=0,'
ant it get a huge number, but not the value of rows.
value of rows for
EXPLAIN select count(*) from photo where group_id=0 and album_id!=0
?
beside the fact, MySQL should not need to investigate any row at all for
this query, it should satisfy this query from the index, without looking up
any row
so this is a bad example
--
Sebastian
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