Re: Adding skip scan (including MDAM style range skip scan) to nbtree - Mailing list pgsql-hackers

On 9/18/24 20:52, Peter Geoghegan wrote:
> On Wed, Sep 18, 2024 at 7:36 AM Tomas Vondra <tomas@vondra.me> wrote:
>> Makes sense. I started with the testing before before even looking at
>> the code, so it's mostly a "black box" approach. I did read the 1995
>> paper before that, and the script generates queries with clauses
>> inspired by that paper, in particular:
> 
> I think that this approach with black box testing is helpful, but also
> something to refine over time. Gray box testing might work best.
> 
>> OK, understood. If it's essentially an independent issue (perhaps even
>> counts as a bug?) what about correcting it on master first? Doesn't
>> sound like something we'd backpatch, I guess.
> 
> What about backpatching it to 17?
> 
> As things stand, you can get quite contradictory counts of the number
> of index scans due to irrelevant implementation details from parallel
> index scan. It just looks wrong, particularly on 17, where it is
> reasonable to expect near exact consistency between parallel and
> serial scans of the same index.
> 

Yes, I think backpatching to 17 would be fine. I'd be worried about
maybe disrupting some monitoring in production systems, but for 17 that
shouldn't be a problem yet. So fine with me.

FWIW I wonder how likely is it that someone has some sort of alerting
tied to this counter. I'd bet few people do. It's probably more about a
couple people looking at explain plans, but they'll be confused even if
we change that only starting with 17.

>> Seems like a bit of a mess. IMHO we should either divide everything by
>> nloops (so that everything is "per loop", or not divide anything. My
>> vote would be to divide, but that's mostly my "learned assumption" from
>> the other fields. But having a 50:50 split is confusing for everyone.
> 
> My idea was that it made most sense to follow the example of
> "Buffers:", since both describe physical costs.
> 
> Honestly, I'm more than ready to take whatever the path of least
> resistance is. If dividing by nloops is what people want, I have no
> objections.
> 

I don't have a strong opinion on this. I just know I'd be confused by
half the counters being total and half /loop, but chances are other
people would disagree.

>>> It's worth having skip support (the idea comes from the MDAM paper),
>>> but it's not essential. Whether or not an opclass has skip support
>>> isn't accounted for by the cost model, but I doubt that it's worth
>>> addressing (the cost model is already pessimistic).
>>>
>>
>> I admit I'm a bit confused. I probably need to reread the paper, but my
>> impression was that the increment/decrement is required for skipscan to
>> work. If we can't do that, how would it generate the intermediate values
>> to search for? I imagine it would be possible to "step through" the
>> index, but I thought the point of skip scan is to not do that.
> 
> I think that you're probably still a bit confused because the
> terminology in this area is a little confusing. There are two ways of
> explaining the situation with types like text and numeric (types that
> lack skip support). The two explanations might seem to be
> contradictory, but they're really not, if you think about it.
> 
> The first way of explaining it, which focuses on how the scan moves
> through the index:
> 
> For a text index column "a", and an int index column "b", skip scan
> will work like this for a query with a qual "WHERE b = 55":
> 
> 1. Find the first/lowest sorting "a" value in the index. Let's say
> that it's "Aardvark".
> 
> 2. Look for matches "WHERE a = 'Aardvark' and b = 55", possibly
> returning some matches.
> 
> 3. Find the next value after "Aardvark" in the index using a probe
> like the one we'd use for a qual "WHERE a > 'Aardvark'". Let's say
> that it turns out to be "Abacus".
> 
> 4. Look for matches "WHERE a = 'Abacus' and b = 55"...
> 
> ... (repeat these steps until we've exhaustively processed every
> existing "a" value in the index)...

Ah, OK. So we do probe the index like this. I was under the impression
we don't do that. But yeah, this makes sense.

> 
> The second way of explaining it, which focuses on how the skip arrays
> advance. Same query (and really the same behavior) as in the first
> explanation:
> 
> 1. Skip array's initial value is the sentinel -inf, which cannot
> possibly match any real index tuple, but can still guide the search.
> So we search for tuples "WHERE a = -inf AND b = 55" (actually we don't
> include the "b = 55" part, since it is unnecessary, but conceptually
> it's a part of what we search for within _bt_first).
> 
> 2. Find that the index has no "a" values matching -inf (it inevitably
> cannot have any matches for -inf), but we do locate the next highest
> match. The closest matching value is "Aardvark". The skip array on "a"
> therefore advances from -inf to "Aardvark".
> 
> 3. Look for matches "WHERE a = 'Aardvark' and b = 55", possibly
> returning some matches.
> 
> 4. Reach tuples after the last match for "WHERE a = 'Aardvark' and b =
> 55", which will cause us to advance the array on "a" incrementally
> inside _bt_advance_array_keys (just like it would if there was a
> standard SAOP array on "a" instead). The skip array on "a" therefore
> advances from "Aardvark" to "Aardvark" +infinitesimal (we need to use
> sentinel values for this text column, which lacks skip support).
> 
> 5. Look for matches "WHERE a = 'Aardvark'+infinitesimal and b = 55",
> which cannot possibly find matches, but, again, can reposition the
> scan as needed. We can't find an exact match, of course, but we do
> locate the next closest match -- which is "Abacus", again. So the skip
> array now advances from "Aardvark" +infinitesimal to "Abacus". The
> sentinel values are made up values, but that doesn't change anything.
> (And, again, we don't include the "b = 55" part here, for the same
> reason as before.)
> 
> 6. Look for matches "WHERE a = 'Abacus' and b = 55"...
> 
> ...(repeat these steps as many times as required)...
> 

Yeah, this makes more sense. Thanks.

> In summary:
> 
> Even index columns that lack skip support get to "increment" (or
> "decrement") their arrays by using sentinel values that represent -inf
> (or +inf for backwards scans), as well as sentinels that represent
> concepts such as "Aardvark" +infinitesimal (or "Zebra" -infinitesimal
> for backwards scans, say). This scheme sounds contradictory, because
> in one sense it allows every skip array to be incremented, but in
> another sense it makes it okay that we don't have a type-specific way
> to increment values for many individual types/opclasses.
> 
> Inventing these sentinel values allows _bt_advance_array_keys to reason about
> arrays without really having to care about which kinds of arrays are
> involved, their order relative to each other, etc. In a certain sense,
> we don't really need explicit "next key" probes of the kind that the
> MDAM paper contemplates, though we do still require the same index
> accesses as a design with explicit accesses.
> 
> Does that make sense?
> 

Yes, it does. Most of my confusion was caused by my belief that we can't
probe the index for the next value without "incrementing" the current
value, but that was a silly idea.

> Obviously, if we did add skip support for text, it would be very
> unlikely to help performance. Sure, one can imagine incrementing from
> "Aardvark" to "Aardvarl" using dedicated opclass infrastructure, but
> that isn't very helpful. You're almost certain to end up accessing the
> same pages with such a scheme, anyway. What are the chances of an
> index with a leading text column actually containing tuples matching
> (say) "WHERE a = 'Aardvarl' and b = 55"? The chances are practically
> zero. Whereas if the column "a" happens to use a discrete type such as
> integer or date, then skip support is likely to help: there's a decent
> chance that a value generated by incrementing the last value
> (and I mean incrementing it for real) will find a real match when
> combined with the user-supplied "b" predicate.
> 
> It might be possible to add skip support for text, but there wouldn't
> be much point.
> 

Stupid question - so why does it make sense for types like int? There
can also be a lot of values between the current and the next value, so
why would that be very different from "incrementing" a text value?

> 
>> I think it'd help if I go through the results and try to prepare some
>> reproducers, to make it easier for you. After all, it's my script and
>> you'd have to reverse engineer some of it.
> 
> Yes, that would be helpful.
> 
> I'll probably memorialize the problem by writing my own minimal test
> case for it. I'm using the same TDD approach for this project as was
> used for the related Postgres 17 project.
> 

Sure. Still, giving you a reproducer should make it easier .

>> This is on exactly the same data, after dropping caches and restarting
>> the instance. So there should be no caching effects. Yet, there's a
>> pretty clear difference - the total number of buffers is the same, but
>> the patched version has many more hits. Yet it's slower. Weird, right?
> 
> Yes, it's weird. It seems likely that you've found an unambiguous bug,
> not just a "regular" performance regression. The regressions that I
> already know about aren't nearly this bad. So it seems like you have
> the right general idea about what to expect, and it seems like your
> approach to testing the patch is effective.
> 

Yeah, it's funny. It's not the first time I start stress testing a patch
only to stumble over some pre-existing issues ... ;-)


regards

-- 
Tomas Vondra