Re: what does "index map: N" mean?

At 2:20 PM +0200 8/30/07, Hartmut Holzgraefe wrote:
>Baron Schwartz wrote:
>>Jay Pipes wrote:
>>>This is very interesting. Sergey, is this engine-agnostic? I
>>>know that Falcon has sparse bitmap indexing (already?) but I did
>>>not know if other engines support the concept (if not, are we
>>>"emulating" it in the optimizer?)
>>>
>>>Baron, which engine are you testing against?
>>
>>I don't think it's the same thing as bitmap indexing -- as I
>>understand it, it's a bitmap that says which indexes which could
>>help a join between two tables, i.e. a cryptic way of listing
>>indexes. At each row in table N, the server looks at the indexes
>>in table N+1 and decides which one might help find matching rows
>>for the current values.
>>
>>I think this isn't the same thing as a bitmap index, which I
>>actually don't really understand. But maybe this is an "emulation"
>>of bitmap indexes.
>
>given what is documented on
>
> http://dev.mysql.com/doc/refman/5.0/en/explain.html
>
>it looks as if it is just showing a column bitmap for
>the utilized index:
>
> # range checked for each record (index map: N)
>
> MySQL found no good index to use, but found that some of indexes might
> be used after column values from preceding tables are known. For each
> row combination in the preceding tables, MySQL checks whether it is
> possible to use a range or index_merge access method to retrieve rows.
> This is not very fast, but is faster than performing a join with no
> index at all. The applicability criteria are as described in Section 6.2.5,
> "Range Optimization", and Section 6.2.6, "Index Merge Optimization",
> with the exception that all column values for the preceding table are
> known and considered to be constants.
>
>looks as if the documentation needs to go into more detail here

And when a dev supplies the information that should be added, we'll be happy to do so.

-- 
Paul DuBois, MySQL Documentation Team
Madison, Wisconsin, USA
MySQL AB, www.mysql.com

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