Re: Combine Prune and Freeze records emitted by vacuum - Mailing list pgsql-hackers

On 29/03/2024 07:04, Melanie Plageman wrote:
> On Thu, Mar 28, 2024 at 11:07:10AM -0400, Melanie Plageman wrote:
>> These comments could use another pass. I had added some extra
>> (probably redundant) content when I thought I was refactoring it a
>> certain way and then changed my mind.
>>
>> Attached is a diff with some ideas I had for a bit of code simplification.
>>
>> Are you working on cleaning this patch up or should I pick it up?
> 
> Attached v9 is rebased over master. But, more importantly, I took
> another pass at heap_prune_chain() and am pretty happy with what I came
> up with. See 0021. I simplified the traversal logic and then grouped the
> chain processing into three branches at the end. I find it much easier
> to understand what we are doing for different types of HOT chains.

Ah yes, agreed, that's nicer.

The 'survivor' variable is a little confusing, especially here:

    if (!survivor)
    {
        int            i;

        /*
         * If no DEAD tuple was found, and the root is redirected, mark it as
         * such.
         */
        if ((i = ItemIdIsRedirected(rootlp)))
            heap_prune_record_unchanged_lp_redirect(prstate, rootoffnum);

        /* the rest of tuples in the chain are normal, unchanged tuples */
        for (; i < nchain; i++)
            heap_prune_record_unchanged(dp, prstate, chainitems[i]);
    }

You would think that "if(!survivor)", it means that there is no live 
tuples on the chain, i.e. no survivors. But in fact it's the opposite; 
it means that the are all live. Maybe call it 'ndeadchain' instead, 
meaning the number of dead items in the chain.

> I got rid of revisited. We can put it back, but I was thinking: we stash
> all HOT tuples and then loop over them later, calling record_unchanged()
> on the ones that aren't marked. But, if we have a lot of HOT tuples, is
> this really that much better than just looping through all the offsets
> and calling record_unchanged() on just the ones that aren't marked?

Well, it requires looping through all the offsets one more time, and 
unless you have a lot of HOT tuples, most items would be marked already. 
But maybe the overhead is negligible anyway.

> I've done that in my version. While testing this, I found that only
> on-access pruning needed this final loop calling record_unchanged() on
> items not yet marked. I know we can't skip this final loop entirely in
> the ON ACCESS case because it calls record_prunable(), but we could
> consider moving that back out into heap_prune_chain()? Or what do you
> think?

Hmm, why is that different with on-access pruning?

Here's another idea: In the first loop through the offsets, where we 
gather the HTSV status of each item, also collect the offsets of all HOT 
and non-HOT items to two separate arrays. Call heap_prune_chain() for 
all the non-HOT items first, and then process any remaining HOT tuples 
that haven't been marked yet.

> I haven't finished updating all the comments, but I am really interested
> to know what you think about heap_prune_chain() now.

Looks much better now, thanks!

-- 
Heikki Linnakangas
Neon (https://neon.tech)