PostgreSQL 10 High Performance - Third Edition

Fill factor

When you create a new index, not every entry in every index block is used. A small amount of free space, specified by the FILLFACTOR parameter, is left empty. The idea is that the first set of changes to that index either updates or insertions can happen on the same index blocks, therefore reducing index fragmentation.

The default FILLFACTOR for B-tree indexes is 90%, leaving 10% free space. One situation where you might want to change this is a table with static data, where the data won't change after index creation. In this case, creating the index to be 100% full is more efficient:

CREATE INDEX i ON t(v) WITH (FILLFACTOR=100);  

On tables that are being populated randomly, or ones that are heavily updated, reducing the fillfactor ...