Why vacuum_index_cleanup is needed for TOAST relations?

I am importing recent changes into my reloption patch and came to a question, 
I did not find an answer...

vacuum_index_cleanup option exists for both heap and toast relations.

As I understand from documentation index cleanup is about is about reporting 
access method that some tuples in table that were indexed are dead, and should 
be cleaned.

And as far as I get, we do not index any TOAST tuples directly. They are 
obtained by getting relation tuple, and then deTOAST it.

So I do not understand why do we need vacuum_index_cleanup for TOAST tables. 
May be we should remove it from there??

Or if I am wrong, can you explain where it is needed?


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