Re: BUG #18205: Performance regression with NOT NULL checks.

PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> I found out that selecting from a wide table with a single not-null where
> clause leads to severe performance regression when upgrading from PostgreSQL
> 9.5 to PostgreSQL 15.

I spent some time poking into this. "git bisect" pins the blame on

commit b8d7f053c5c2bf2a7e8734fe3327f6a8bc711755
Author: Andres Freund <andres(at)anarazel(dot)de>
Date: Tue Mar 14 15:45:36 2017 -0700

Faster expression evaluation and targetlist projection.

The time needed for the seqscan gets about 50% worse at that
commit (on my test machine, anyway):

-> Seq Scan on testtable t (cost=0.00..51892.80 rows=9730 width=4) (actual time=0.010..204.937 rows=6000000 loops=1)
Filter: (y IS NULL)

versus

-> Seq Scan on testtable t (cost=0.00..51892.80 rows=9730 width=4) (actual time=0.013..317.069 rows=6000000 loops=1)
Filter: (y IS NULL)

"perf" says that the extra time is mostly spent in slot_deform_tuple's
inner loop:

for (; attnum < natts; attnum++)
{
Form_pg_attribute thisatt = att[attnum];

if (hasnulls && att_isnull(attnum, bp))
{
values[attnum] = (Datum) 0;
isnull[attnum] = true;
slow = true; /* can't use attcacheoff anymore */
continue;
}
...

which confused me, because that code doesn't look materially different
in v10 than 9.6. I eventually realized that the reason is that we
reach slot_deform_tuple with natts = 26 in the new code, but in the
old code we do not, thanks to this short-circuit in slot_getattr:

/*
* check if target attribute is null: no point in groveling through tuple
*/
if (HeapTupleHasNulls(tuple) && att_isnull(attnum - 1, tup->t_bits))
{
*isnull = true;
return (Datum) 0;
}

So that results in not having to deconstruct most of the tuple,
whereas in the new code we do have to, thanks to b8d7f053c's
decision to batch all the variable-value-extraction work.

This is a pretty narrow corner case: it would only matter if the
column you're testing for null-ness is far past any other column
the query needs to fetch. So I'm not sure that it's worth doing
anything about. You could imagine special processing for
NullTest-on-a-simple-Var: exclude the Var from the ones that we
extract normally and instead compile it into some sort of "is column k
NULL" test on the HeapTuple. But that seems messy, and it could be a
significant pessimization for storage methods that aren't like heapam.

On the whole I'm inclined to say "sorry, that's the price of
progress". But it is a bit sad that a patch intended to be a
performance win shows a loss this big on a (presumably) real-world
case.

regards, tom lane