Re: [PERFORM] Query planner gaining the ability to replanning afterstart of query execution.

> Can you be more elaborate how you'd want to go about it?

My initial approach would be to try to identify places in the plan
where selectivity is seriously over or underestimated.   I would reuse
the instrumentation infrastructure's counts of filtered and returned
tuples for each execnode, and periodically call back into the planner
(for example at every power of 2 tuples processed).

The planner would have a wrapper to clauselist_selectivity which
somehow combines the existing estimate with the filtered and returned
tuples so far.   Exactly how to merge them isn't clear, but I could
imagine using a poisson distribution to calculate the probability that
the selectivity estimate is representative of the filtered and
returned numbers, and then blending the two linearly based on that
estimate.

When the planner has re-estimated the cost of the current plan, a
discount would be applied for the percentage of each execnode
completed (rows processed / estimated rows), and all candidate plans
compared.

If another candidate plan is now lower cost, the current plan would be
terminated[1] by setting a flag instructing each execnode to return as
if it had reached the end of the input, although still caching the
node selectivity values, and the new plan started from scratch.

The old plan is kept within the query planner candidate list, together
with it's cached selectivity values.  If at some point it again is
cheaper, it is started from scratch too.


> Even if we know the cardinality is overestimated, we have no idea whether the cardinality of a = 3 or b = 40 is wrong
orthey just correlate 

The goal would be not to know which is wrong, but to try each,
discarding it if it turns out worse than we estimated.  Processing a
few hundred rows of each of 5 plans is tiny compared to a scan of 1M
rows...


[1]:   An improvement here (with much more code complexity) is to keep
multiple partially executed plans around, so that whichever one is
most promising can be worked on, but can be halted and resumed later
as selectivity (and hence cost) estimates change.

On Mon, Nov 13, 2017 at 8:06 PM, Arne Roland <A.Roland@index.de> wrote:
> Hello,
>
> I'd love to have some sort of dynamic query feedback, yet it's very complicated to do it right. I am not convinced
thatchanging the plan during a single execution is the right way to do it, not only because it sounds intrusive to do
crazythings in the executor, but also because don't understand why the new plan should be any better than the old one.
Canyou be more elaborate how you'd want to go about it? 
>
> In your example (which presumably just has a single relation), we have no notion of whether the scan returns no rows
becausewe were unlucky, because just the first few pages were empty of matching rows (which in my experience happens
moreoften), or because the cardinality estimation is wrong. Even if the cardinality estimation is wrong, we have no
notionof which predicate or predicate combination actually caused the misestimation. If the first few pages where
empty,the same might happen with every order (so also with every available indexscan). Imagine a very simple seqscan
planof 
> select * from mytab where a = 3 and b = 40 limit 1
> Even if we know the cardinality is overestimated, we have no idea whether the cardinality of a = 3 or b = 40 is wrong
orthey just correlate, so there is no notion of which is actually the cheapest plan. Usual workaround for most of these
queriesis to add an order by (which has the nice addition of having a deterministic result) with an appropriate complex
index,usually resulting in indexscans. 
>
> While we actually know more after the first execution of a nodes like materialize, sort or hash nodes, I rarely
encountermaterialize nodes in the wild. Consequently that is the place where the work is usually already done, which is
especiallytrue with the hash node. Even though it still might be more optimal to switch from a mergejoin to a hashjoin
insome cases, I doubt that's worth any work (and even less the maintenance). 
>
> Best regards
> Arne Roland
>
> -----Original Message-----
> From: pgsql-performance-owner@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Oliver
Mattos
> Sent: Monday, November 13, 2017 5:45 PM
> To: pgsql-performance@postgresql.org
> Subject: [PERFORM] Query planner gaining the ability to replanning after start of query execution.
>
> I am interested in giving the query planner the ability to replan (or re-rank plans) after query execution has begun,
basedon the progression of the query so far. 
>
> Example use case:
>
> *  A LIMIT 1 query is planned using an expensive scan which the planner expects to return a large number of results,
andto terminate 
> early.   The reality is the query actually produces no results, and
> the scan must run to completion, potentially taking thousands of times longer than expected.
>
> *  If this plans costs were adjusted mid-execution to reflect the fact that the scan is producing far fewer rows than
expected,then another query plan might come out ahead, which would complete far faster. 
>
>
> Has this been done before?   Are there any pitfalls to beware of?
>
>
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>
>
>
> -----Original Message-----
> From: pgsql-performance-owner@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Oliver
Mattos
> Sent: Monday, November 13, 2017 5:45 PM
> To: pgsql-performance@postgresql.org
> Subject: [PERFORM] Query planner gaining the ability to replanning after start of query execution.
>
> I am interested in giving the query planner the ability to replan (or re-rank plans) after query execution has begun,
basedon the progression of the query so far. 
>
> Example use case:
>
> *  A LIMIT 1 query is planned using an expensive scan which the planner expects to return a large number of results,
andto terminate 
> early.   The reality is the query actually produces no results, and
> the scan must run to completion, potentially taking thousands of times longer than expected.
>
> *  If this plans costs were adjusted mid-execution to reflect the fact that the scan is producing far fewer rows than
expected,then another query plan might come out ahead, which would complete far faster. 
>
>
> Has this been done before?   Are there any pitfalls to beware of?
>
>
> --
> Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-performance
>
>
>


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