Explain explanation (was: More LEFT JOIN newbie fun!) - 05-11-2006 , 12:03 PM
Following my post about this complex search I'm trying to do...
In the initial post I said I'd tried adding:
left join table_ga as tga on tga.id = r.id
left join table_a as ta on ta.ida = tga.ida
left join (table_ga as tga inner join table_a as ta) on
(tga.id = r.id and ta.ida = tga.ida)
to my query, and that both got the same results. Having added two
pairs and tried both versions, I find the second one is fractionally
(but I mean /really/ fractionally) faster, as reported by phpMyAdmin.
However, when I do an Explain on both versions, the first shows
values of 375, 17, 3 and a bunch of 1s in the rows column, whereas
the second shows 375, 34, 6 and the same lot of 1s. All other details
As I understand it in my naive, newbie way, as a rough rule of thumb
you can compare the speeds of queries by multiplying together the
rows values... in which case the search that was fractionally slower
should have been four times as fast. What's going on here?
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