It means basically exporting your database (or tables) and importing it again. What happens is that deleted data which doesn't necessarily free up space (Oracle reuses the freed up space sometimes) doesn't get exported.
https://www.iri.com/blog/vldb-operations/database-reorgs-why...
https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUEST...
Is there a way to just do the index build part, short of dropping an index and adding it back?
> REINDEX INDEX CONCURRENTLY index_name;
(A vanilla REINDEX will lock the table, preventing writes, while it runs. The CONCURRENT creates a new index, replicates any updates to the original while it does so, and then does an atomic switcheroo at the end.)
I used PostgreSQL fairly recently (a year or so ago?) and ended up abandoning it after I was forced to do the export/import dance through a few version upgrades.
When did that requirement go away?
The difference is that you can use logical replication since 10 to prevent downtime during upgrade.
Which if you were using it a year ago could have been done.
This makes me very nervous tho, I've at least two exts (trigrams and gists) maybe they work, maybe not, I just prefer the ease of mind of a old fashioned dump.
It'd be something like nominatim.so, which is external to PGSQL and (AFAIK) has its own format for certain types of data and indexes.
Dump and reload is ok if you have a small database or can afford hours of downtime... if not, use pg_upgrade.