Hi Kelvin, all!
Kelvin Wu wrote:
I am looking for help and suggestion.
Mysql Ver 12.18 Distrib 4.0.12, for pc-linux (i686)
The table structure:
[[...]]
I created multiple parts of Index for this table:
[[...]]
And here are the questions:
Q1: the query is supposed to use index bynone but ...
[[...]]
Q2: the query is supposed to use index bysource but ...
[[...]]
Using of "timestamp > 0" caused using different index on different source =
xxx... why is like that so?
Q3: the query is supposed to use index bycategory and it seems work fine so
far...
[[...]]
Any idea?
Very recently, there was another posting about an index not being used
for a query, and the explanation was:
If a "where condition" is matched by too many records in the table,
the index is *intentionally* not used,
because going through the index and then accessing (nearly) all the
rows is more work than just scanning the table (and finding the many
hits).
Remember that acessing a row is the main cost, and going through the
index adds work. Doing that additional work only pays off if it saves
accessing a sufficient number of rows. Check the row counts in your
examples.
Please scan the list archive to find the discussion and the detailed
references.
HTH,
Jörg
--
Joerg Bruehe, Senior Production Engineer
MySQL AB, www.mysql.com
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