Re: Memory-Bounded Hash Aggregation

On Wed, 2020-02-19 at 20:16 +0100, Tomas Vondra wrote:
> 1) explain.c currently does this:
> 
> I wonder if we could show something for plain explain (without
> analyze).
> At least the initial estimate of partitions, etc. I know not showing
> those details until after execution is what e.g. sort does, but I
> find
> it a bit annoying.

Looks like you meant to include some example explain output, but I
think I understand what you mean. I'll look into it.

> 2) The ExecBuildAggTrans comment should probably explain "spilled".

Done.

> 3) I wonder if we need to invent new opcodes? Wouldn't it be simpler
> to
> just add a new flag to the agg_* structs instead? I haven't tried
> hacking
> this, so maybe it's a silly idea.

There was a reason I didn't do it this way, but I'm trying to remember
why. I'll look into this, also.

> 4) lookup_hash_entries says
> 
>    /* check to see if we need to spill the tuple for this grouping
> set */
> 
> But that seems bogus, because AFAIK we can't spill tuples for
> grouping
> sets. So maybe this should say just "grouping"?

Yes, we can spill tuples for grouping sets. Unfortunately, I think my
tests (which covered this case previously) don't seem to be exercising
that path well now. I am going to improve my tests, too.

> 5) Assert(nbuckets > 0);

I did not repro this issue, but I did set a floor of 256 buckets.

> which fails with segfault at execution time:

Fixed. I was resetting the hash table context without setting the
pointers to NULL.

Thanks! I attached a new, rebased version. The fixes are quick fixes
for now and I will revisit them after I improve my test cases (which
might find more issues).

Regards,
    Jeff Davis