Me & Greg just had a little chat, and came up with this scheme: 1. on heap_update, if the page is full, you prune the page (but don't defragment it, because you can't get the vacuum lock). That hopefully leaves behind a large enough gap to put the new tuple in. Insert the new tuple in the gap, and mark the page as Fragmented. Also make a note in some backend-private data structure that we've left that page in fragmented state.
2. In UnpinBuffer, if the pin count falls to zero and it's a page we've pruned (check the backend-private data structure), defragment it. Under little contention, all the cost of pruning will be carried by transactions that do updates. Whether we need to prune in heap_fetch in addition to that to keep the chains short, I don't know. One problem with this scheme is that when the page gets full, you have to hope that you can create a wide enough gap by pruning. It will work well with fixed-size tuples, but not so well otherwise. Hmm. I wonder if we could prune/defragment in bgwriter? -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com ---------------------------(end of broadcast)--------------------------- TIP 4: Have you searched our list archives? http://archives.postgresql.org
