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

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.

This seems to work on all engines AFAIK.

> On a related note, I have been testing performance regressions of
> subqueries in 5.2.6 (yes, I wrote regressions, not improvements...) and
> have noticed another EXPLAIN phrase:
>
> "Using index condition"
>
> Sergey, what does this mean?
>
> For reference, see the bug report I filed here:
>
> http://bugs.mysql.com/?id=30659

Hmmm, as soon as anyone says what this means I'll add it to the visual explain tool. It seems there are a lot of undocumented things in EXPLAIN.

Baron

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