indexing objectclass: why?

[Roel van Meer <rolek@linvision.com>]

Hi list,

I had some trouble with the performance of my openldap 2.0.18. The
search performance was lousy.
The database has about 200.000 entries, and i do have indexes for
the attributes searched on.

In the Faq-o-matic, (http://www.openldap.org/faq/data/cache/42.html)
i found:

> If, after you add any indexes your slapd still appears to be really slow,
> try adding an equality index for objectClass.

This helped. search times went down from 47 seconds to about 0.02
seconds.
Can anyone explain to me _why_ this influences performance so much?

I do not do any searching on objectclass, so it seemed to me it
wasn't neccesary to add an index for it, but apparently, it is.

If this index is needed in all situations, maybe put an entry in the
manpage, or the admin guide?

Regards,
rolek
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