We know that HOT can cause line pointer bloat because of redirect dead line pointers. In the worst case there could be MaxHeapTuplesPerPage redirect-dead line pointers in a page. VACUUM can reclaim these line pointers and mark them ~LP_USED (what is now called LP_UNUSED). But we don't reclaim the space used by unused line pointers during repairing page fragmentation, and hence we would never be able to remove the line pointer bloat completely. Fundamentally we should be able to reclaim the unused line pointers at the end of the lp array (i.e. unused line pointers immediate to pd_lower)
I had earlier tried to repair the bloat by reclaiming the space used by LP_UNUSED line pointers at the end of the array. But it doesn't work well with VACUUM FULL which tracks unused line pointers for moving tuples. Its not that we can not fix that issue, but I am reluctant to spend time on that right now because many of us feel that VACUUM FULL is near its EOL. How about passing a boolean to PageRepairFragmentation to command it to reclaim unused line pointers ? We pass "true" at all places except in the VACUUM FULL code path. IOW we reclaim unused line pointers in defragmentation and LAZY VACUUM. We would need to WAL log this information in xl_heap_clean so that we redo the same during recovery. I have a patch ready since I had already implemented this few weeks back. Comments ? Thanks, Pavan -- Pavan Deolasee EnterpriseDB http://www.enterprisedb.com
