Thanks for your answer, Klas.
That makes sense. But did you mean to play around with the indexes even on core joomla or seblod tables (e.g. #__contentitem_tag_map)?
I was also thinking that it would be really great if search join field was conrolled by restriction rules (e.g. enabled only when the tag field is selected) but i'm not lucky to apply the restriction rules on search join table field.
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The last option I see is to create own custom SEBLOD field plugin used in the search form with onCCK_FieldPrepareSearch function, which would manage to join the table only when the value of the field is selected in the form.
Can you please shortly comment on the restriction possibility?
Thanks
Michal