Re: Why the planner does not use index for a large amount of data?

​Please avoid posting to multiple lists at once.

An index doesn't contain visibility information so every record located in the index must also be checked on the storage table to determine if it is visible to the current transaction and thus is valid to be returned.  If it does need to be returned the table itself is also needed to get the rest of the information.  Thus the index scan itself involves additional non-value-added (NVA) effort so far as the query is concerned.  If a large fraction (I've seen estimates of 10%) of the table is estimated to be returned the additional cost involved with scanning the entire table will less than the NVA cost of walking through the index and then fetching records from the table anyway.

Also: "Fetching rows separately is much more expensive than reading them sequentially."