(This mostly pertains to recreating a PK on an existing table.)
Is there any reason to have the PK index on an ever-increasing field (for
example SERIAL, sequence or timestamp fed by clock_timestamp() at time zone
'UTC') be anything but fillfactor=100?
New records will always be added to the "lower right hand corner" of the
tree, so having 20% empty space in the rest of the tree would just waste
space (mainly buffers, but disk space could even start to add up on Very Big
Tables).
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Born in Arizona, moved to Babylonia.