> The background on index-only scans:
>
> https://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.2#Index-only_scans
>
>
> In either case(index, index-only) the index has to be scanned. The
> difference is where the data is pulled from. In the index-only scan
> the query still needs to consult the visibility map to determine
> whether the tuple pointed to by the index entry is visible. I would
> say that in the limit 1 case the planner determines it is just as easy
> to check and pull the data from the actual tuple as to to check the
> visibility map. In the limit 100 case it becomes more cost effective
> to use the visibility map and pull data directly from the index data.
>
>
Thank you for your answer
Enrico
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