"Heikki Linnakangas" <heikki@enterprisedb.com> writes:
> Tom Lane wrote:
>> I think we really are at too much risk of PANIC the way it's being done
>> now. Has anyone got a better idea?
> We could do the pruning in two phases: first figure out what to do
> without modifyng anything, outside critical-section, and then actually
> do it, inside critical section.
> Looking at heap_page_prune, we already collect information of what we
> did in the redirected/nowdead/nowunused arrays for WAL logging purposes.
That's a thought, but ...
> We could use that, but we would also have to teach heap_prune_chain to
> not step into tuples that we've already decided to remove.
... seems like this would require searching the aforementioned arrays
for each tuple examined, which could turn into an O(N^2) problem.
If there are many removable tuples it could easily end up slower than
copying.
[ thinks some more... ] I guess we could use a flag array dimensioned
MaxHeapTuplesPerPage to mark already-processed tuples, so that you
wouldn't need to search the existing arrays but just index into the flag
array with the tuple's offsetnumber.
I wonder if the logic could be restructured to avoid this by taking
advantage of it being a two-pass process, instead of fighting it?
But that'd probably be a bigger change than we'd want to risk
back-patching.
Since I'm the one complaining about the PANIC risk, I guess I should
do the legwork here.
regards, tom lane