On 9/8/16 3:48 AM, Masahiko Sawada wrote:
If we replaced dead_tuples with an array-of-array, isn't there negative performance impact for lazy_tid_reap()? As chunk is added, that performance would be decrease.
Yes, it certainly would, as you'd have to do 2 binary searches. I'm not sure how much that matters though; presumably the index scans are normally IO-bound?
Another option would be to use the size estimation ideas others have mentioned to create one array. If the estimates prove to be wrong you could then create a single additional segment; by that point you should have a better idea of how far off the original estimate was. That means the added search cost would only be a compare and a second pointer redirect.
Something else that occurred to me... AFAIK the only reason we don't support syncscan with VACUUM is because it would require sorting the TID list. If we just added a second TID list we would be able to support syncscan, swapping over to the 'low' list when we hit the end of the relation.
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