The attached proof of concept patch greatly improves the bloat for both the insert and the update cases. You need to turn on both features: adding the pages to fsm, and vacuuming the fsm, to get the benefit (so JJ_GIN=3). The first of those two things could probably be adopted for real, but the second probably is not acceptable. What is the right way to do this? Could a variant of RecordFreeIndexPage bubble the free space up the map immediately rather than waiting for a vacuum? It would only have to move up until it found a page with freespace already recorded in it, which the vast majority of the time would mean observing up one level and then not writing to it, assuming the pending list pages remain well clustered.
You make a good case for action here since insert only tables with GIN indexes on text are a common use case for GIN.
Why would vacuuming the FSM be unacceptable? With a large gin_pending_list_limit it makes sense.
If it is unacceptable, perhaps we can avoid calling it every time, or simply have FreeSpaceMapVacuum() terminate more quickly on some kind of 80/20 heuristic for this case.
Couple of questions here...
* the docs say "it's desirable to have pending-list cleanup occur in the background", but there is no way to invoke that, except via VACUUM. I think we need a separate function to be able to call this as a background action. If we had that, we wouldn't need much else, would we?
* why do we have two parameters: gin_pending_list_limit and fastupdate? What happens if we set gin_pending_list_limit but don't set fastupdate?
* how do we know how to set that parameter? Is there a way of knowing gin_pending_list_limit has been reached?
This and the OP seem like 9.5 open items to me.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
