System catalog indexes do not support deduplication as a matter of policy. I chose to do things that way during the Postgres 13 development cycle due to the restriction on using storage parameters with system catalog indexes. At the time I felt that *forcing* the use of deduplication with system catalog indexes might expose users to problems. But this is something that seems worth revisiting now. (I haven't actually investigated what it would take to make system catalogs support the 'deduplicate_items' parameter, but that may not matter now.)
I would like to enable deduplication within system catalog indexes for Postgres 15. Leaving it disabled forever seems kind of arbitrary at best. In general enabling deduplication (or not disabling it) has only a fixed, small downside in the worst case. It has a huge upside in favorable cases. Deduplication is part of our high level strategy for avoiding nbtree index bloat from version churn (non-HOT updates with several indexes that are never "logically modified"). It effectively cooperates with and enhances the new enhancements to index deletion in Postgres 14. Plus these recent index deletion enhancements more or less eliminated a theoretical downside of deduplication: now it doesn't really matter that posting list tuples only have a single LP_DEAD bit (if it ever did). This is because we can now do granular posting list TID deletion, provided the deletion process visits the same heap block in passing. I can find no evidence that even one single user found it useful to disable deduplication while using Postgres 13 in production (by searching for "deduplicate_items" on Google). While I myself said that there might be a regression of up to 2% of throughput back in early 2020, that was under highly unrealistic conditions, that could never apply to system catalogs -- I was being conservative. Most system catalog indexes are unique indexes, where there is no possible overhead from deduplication unless we already know for sure that the index is subject to some kind of version churn (and so have high confidence that deduplication will be at least somewhat effective at buying time for VACUUM). The non-unique system catalog indexes seem pretty likely to benefit from deduplication in the usual obvious way (not so much because of versioning and bloat). The two pg_depend non-unique indexes tend to have a fair number of duplicates. -- Peter Geoghegan
