Обсуждение: BRIN index which is much faster never chosen by planner

On Thu, Oct 10, 2019 at 04:58:11PM -0500, Jeremy Finzel wrote:
>
> ...
>
>Notice it chooses the smallest BRIN index with 1000 pages per range, and
>this is far faster than the seq scan.
>
>I do believe the estimate is actually way off.  Just a plain EXPLAIN of the
>latter estimates 10x more rows than actual:
> WindowAgg  (cost=24354689.19..24354705.07 rows=706 width=120)
>   ->  Sort  (cost=24354689.19..24354690.95 rows=706 width=104)
>         Sort Key: unique_cases.source, unique_cases.rec_insert_time
>         ->  Subquery Scan on unique_cases  (cost=24354641.66..24354655.78
>rows=706 width=104)
>               ->  Unique  (cost=24354641.66..24354648.72 rows=706
>width=124)
>                     ->  Sort  (cost=24354641.66..24354643.42 rows=706
>width=124)
>                           Sort Key: l.brand_id, l.last_change, l.log_id,
>l.rec_insert_time DESC
>                           ->  Nested Loop  (cost=2385.42..24354608.25
>rows=706 width=124)
>                                 ->  Bitmap Heap Scan on log_table l
> (cost=2383.82..24352408.26 rows=706 width=99)
>                                       Recheck Cond: (rec_insert_time >=
>(now() - '10 days'::interval))
>                                       Filter: ((field1 IS NOT NULL) AND
>(category = 'music'::name))
>                                       ->  Bitmap Index Scan on
>rec_insert_time_brin_1000  (cost=0.00..2383.64 rows=156577455 width=0)
>                                             Index Cond: (rec_insert_time
>>= (now() - '10 days'::interval))
>                                 ->  Bitmap Heap Scan on small_join_table
>filter  (cost=1.60..3.12 rows=1 width=8)
>                                       Recheck Cond: (category =
>(l.category)::text)
>                                       ->  Bitmap Index Scan on
>small_join_table_pkey  (cost=0.00..1.60 rows=1 width=0)
>                                             Index Cond: (category =
>(l.category)::text)
>(17 rows)
>
>
>Here is EXPLAIN only of the default chosen plan:
> WindowAgg  (cost=24437857.18..24437873.07 rows=706 width=120)
>   ->  Sort  (cost=24437857.18..24437858.95 rows=706 width=104)
>         Sort Key: unique_cases.source, unique_cases.rec_insert_time
>         ->  Subquery Scan on unique_cases  (cost=24437809.66..24437823.78
>rows=706 width=104)
>               ->  Unique  (cost=24437809.66..24437816.72 rows=706
>width=124)
>                     ->  Sort  (cost=24437809.66..24437811.42 rows=706
>width=124)
>                           Sort Key: l.brand_id, l.last_change, l.log_id,
>l.rec_insert_time DESC
>                           ->  Nested Loop  (cost=0.00..24437776.25
>rows=706 width=124)
>                                 Join Filter: ((l.category)::text =
>filter.category)
>                                 ->  Seq Scan on log_table l
> (cost=0.00..24420483.80 rows=706 width=99)
>                                       Filter: ((field1 IS NOT NULL) AND
>(category = 'music'::name) AND (rec_insert_time >= (now() - '10
>days'::interval)))
>                                 ->  Materialize  (cost=0.00..33.98
>rows=1399 width=8)
>                                       ->  Seq Scan on small_join_table
>filter  (cost=0.00..26.99 rows=1399 width=8)
>(13 rows)
>
>
>
>Any insight into this is much appreciated.  This is just one example of
>many similar issues I have been finding with BRIN indexes scaling
>predictably with insert-only workloads.
>

It's quite interesting planning issue. The cost estimates are:

 ->  Seq Scan on foo.log_table l
 (cost=0.00..24420483.80 rows=707 width=99) (actual

while for the bitmap heap scan it looks like this:

 ->  Bitmap Heap Scan on foo.log_table l
 (cost=2391.71..24360848.29 rows=735 width=99) (actual

So the planner actualy thinks the bitmap heap scan is a tad *cheaper*
but picks the seq scan anyway. This is likely because we don't really
compare the exact costs, but we do fuzzy comparison - the plan has to be
at least 1% cheaper to dominate the existing plan. This allows us to
save some work when replacing the paths.

In this case the difference is only about 0.2%, so we keep the seqscan
path. The real question is why the planner came to this cost, when it
got pretty good row estimates etc.

Looking at the cost_bitmap_heap_scan() I think the costing issue comes
mostly from this bit:

  /*
   * For small numbers of pages we should charge spc_random_page_cost
   * apiece, while if nearly all the table's pages are being read, it's more
   * appropriate to charge spc_seq_page_cost apiece.  The effect is
   * nonlinear, too. For lack of a better idea, interpolate like this to
   * determine the cost per page.
   */
  if (pages_fetched >= 2.0)
      cost_per_page = spc_random_page_cost -
          (spc_random_page_cost - spc_seq_page_cost)
          * sqrt(pages_fetched / T);
  else
      cost_per_page = spc_random_page_cost;

The index scan is estimated to return 157328135 rows, i.e. about 50% of
the table (apparently it's ~10x more than the actual number). This is
produced by compute_bitmap_pages() which also computes pages_fetched,
and I guess that's going to be pretty close to all pages, because with
T = 18350080 (which is 140GB) and using

  pages_fetched = (2.0 * T * tuples_fetched) / (2.0 * T + tuples_fetched);

we get 29731418 (which is more than 18350080, so it gets clamped).

So this kinda seems like the optimizer kinda believes it'll have to scan
the whole table anyway. In reality, of course, the number of tuples
returned by the index is 10x lower, so the formula above would give us
only about 10975262 pages (so ~1/2 the table). The actual number of
pages is however even lower - only about 1509030, i.e. ~8% of the table.

So this seems like a combination of multiple issues. Firstly, the bitmap
index scan on rec_insert_time_brin_1000 estimate seems somewhat poor. It
might be worth increasing stats target on that column, or something like
that. Not sure, but it's about the only "fixable" thing here, I think.

The other issue is that the estimation of pages fetched using bitmap
heap scan is rather crude - but that's simply hard, and I don't think we
can fundamentally improve this.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services 

On Fri, 11 Oct 2019 at 12:13, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> The index scan is estimated to return 157328135 rows, i.e. about 50% of
> the table (apparently it's ~10x more than the actual number).

Don't pay too much attention to the actual row counts from bitmap
index scans of brin indexes. The value is entirely made up, per:

/*
* XXX We have an approximation of the number of *pages* that our scan
* returns, but we don't have a precise idea of the number of heap tuples
* involved.
*/
return totalpages * 10;

in bringetbitmap().

(Ideally EXPLAIN would be written in such a way that it didn't even
show the actual rows for node types that don't return rows. However,
I'm sure that would break many explain parsing tools)

The planner might be able to get a better estimate on the number of
matching rows if the now() - interval '10 days' expression was
replaced with 'now'::timestamptz - interval '10 days'. However, care
would need to be taken to ensure the plan is never prepared since
'now' is evaluated during parse. The same care must be taken when
creating views, functions, stored procedures and the like.

The planner will just estimate the selectivity of now() - interval '10
days'  by using DEFAULT_INEQ_SEL, which is 0.3333333333333333, so it
thinks it'll get 1/3rd of the table.  Using 'now' will allow the
planner to lookup actual statistics on that column which will likely
give a much better estimate, which by the looks of it, likely will
result in one of those BRIN index being used.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

On Fri, 11 Oct 2019 at 17:48, Michael Lewis <mlewis@entrata.com> wrote:
>
> On Thu, Oct 10, 2019 at 6:22 PM David Rowley <david.rowley@2ndquadrant.com> wrote:
>> The planner will just estimate the selectivity of now() - interval '10
>> days'  by using DEFAULT_INEQ_SEL, which is 0.3333333333333333, so it
>> thinks it'll get 1/3rd of the table.  Using 'now' will allow the
>> planner to lookup actual statistics on that column which will likely
>> give a much better estimate, which by the looks of it, likely will
>> result in one of those BRIN index being used.
>
>
> This surprised me a bit, and would have significant implications. I tested a few different tables in our system and
getthe same row count estimate with either WHERE condition. Perhaps I am missing a critical piece of what you said.
 
>
> explain
> select * from charges where posted_on > now() - interval '10 days';
>
> explain
> select * from charges where posted_on > 'now'::timestamptz  - interval '10 days';

You're right. On looking more closely at the code, it uses
estimate_expression_value(), which performs additional constant
folding of expressions for selectivity purposes only. It does end up
calling the now() function and evaluating the now() - interval '10
days'; expression into a Const.

The header comment for that function reads:

* estimate_expression_value
 *
 * This function attempts to estimate the value of an expression for
 * planning purposes.  It is in essence a more aggressive version of
 * eval_const_expressions(): we will perform constant reductions that are
 * not necessarily 100% safe, but are reasonable for estimation purposes.

So I take back what I said about using 'now'::timestamptz instead of now().

-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

On Fri, Oct 11, 2019 at 09:08:05AM -0500, Jeremy Finzel wrote:
>On Thu, Oct 10, 2019 at 7:22 PM David Rowley <david.rowley@2ndquadrant.com>
>wrote:
>
>> The planner might be able to get a better estimate on the number of
>> matching rows if the now() - interval '10 days' expression was
>> replaced with 'now'::timestamptz - interval '10 days'. However, care
>> would need to be taken to ensure the plan is never prepared since
>> 'now' is evaluated during parse. The same care must be taken when
>> creating views, functions, stored procedures and the like.
>>
>
>You are on to something here I think with the now() function, even if above
>suggestion is not exactly right as you said further down.  I am finding a
>hard-coded timestamp gives the right query plan.  I also tested same with
>even bigger window (last 16 days) and it yet still chooses the brin index.
>
>foo_prod=# EXPLAIN
>foo_prod-# SELECT
>foo_prod-#  category, source, MIN(rec_insert_time) OVER (partition by
>source order by rec_insert_time) AS first_source_time, MAX(rec_insert_time)
>OVER (partition by source order by rec_insert_time) AS last_source_time
>foo_prod-# FROM (SELECT DISTINCT ON (brand_id, last_change, log_id)
>foo_prod(# category, source(field1) AS source, rec_insert_time
>foo_prod(# FROM log_table l
>foo_prod(# INNER JOIN public.small_join_table filter ON filter.category =
>l.category
>foo_prod(# WHERE field1 IS NOT NULL AND l.category = 'music'
>foo_prod(# AND l.rec_insert_time >= now() - interval '10 days'
>foo_prod(# ORDER BY brand_id, last_change, log_id, rec_insert_time DESC)
>unique_cases;
>
>   QUERY PLAN

>-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> WindowAgg  (cost=24436329.10..24436343.56 rows=643 width=120)
>   ->  Sort  (cost=24436329.10..24436330.70 rows=643 width=104)
>         Sort Key: unique_cases.source, unique_cases.rec_insert_time
>         ->  Subquery Scan on unique_cases  (cost=24436286.24..24436299.10
>rows=643 width=104)
>               ->  Unique  (cost=24436286.24..24436292.67 rows=643
>width=124)
>                     ->  Sort  (cost=24436286.24..24436287.85 rows=643
>width=124)
>                           Sort Key: l.brand_id, l.last_change, l.log_id,
>l.rec_insert_time DESC
>                           ->  Nested Loop  (cost=0.00..24436256.25
>rows=643 width=124)
>                                 Join Filter: ((l.category)::text =
>filter.category)
>                                 ->  Seq Scan on small_join_table filter
> (cost=0.00..26.99 rows=1399 width=8)
>                                 ->  Materialize  (cost=0.00..24420487.02
>rows=643 width=99)
>                                       ->  Seq Scan on log_table l
> (cost=0.00..24420483.80 rows=643 width=99)
>                                             Filter: ((field1 IS NOT NULL)
>AND (category = 'music'::name) AND (rec_insert_time >= (now() - '10
>days'::interval)))
>(13 rows)
>
>foo_prod=# SELECT now() - interval '10 days';
>           ?column?
>-------------------------------
> 2019-10-01 08:20:38.115471-05
>(1 row)
>
>foo_prod=# EXPLAIN
>SELECT
> category, source, MIN(rec_insert_time) OVER (partition by source order by
>rec_insert_time) AS first_source_time, MAX(rec_insert_time) OVER (partition
>by source order by rec_insert_time) AS last_source_time
>FROM (SELECT DISTINCT ON (brand_id, last_change, log_id)
>category, source(field1) AS source, rec_insert_time
>FROM log_table l
>INNER JOIN public.small_join_table filter ON filter.category = l.category
>WHERE field1 IS NOT NULL AND l.category = 'music'
>AND l.rec_insert_time >= '2019-10-01 08:20:38.115471-05'
>ORDER BY brand_id, last_change, log_id, rec_insert_time DESC) unique_cases;
>
>  QUERY PLAN

>-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
> WindowAgg  (cost=19664576.17..19664590.63 rows=643 width=120)
>   ->  Sort  (cost=19664576.17..19664577.77 rows=643 width=104)
>         Sort Key: unique_cases.source, unique_cases.rec_insert_time
>         ->  Subquery Scan on unique_cases  (cost=19664533.31..19664546.17
>rows=643 width=104)
>               ->  Unique  (cost=19664533.31..19664539.74 rows=643
>width=124)
>                     ->  Sort  (cost=19664533.31..19664534.92 rows=643
>width=124)
>                           Sort Key: l.brand_id, l.last_change, l.log_id,
>l.rec_insert_time DESC
>                           ->  Nested Loop  (cost=3181.19..19664503.32
>rows=643 width=124)
>                                 ->  Gather  (cost=3180.91..19662574.92
>rows=643 width=99)
>                                       Workers Planned: 3
>                                       ->  Parallel Bitmap Heap Scan on
>log_table l  (cost=2180.91..19661510.62 rows=207 width=99)
>                                             Recheck Cond: (rec_insert_time
>>= '2019-10-01 08:20:38.115471-05'::timestamp with time zone)
>                                             Filter: ((field1 IS NOT NULL)
>AND (category = 'music'::name))
>                                             ->  Bitmap Index Scan on
>rec_insert_time_brin_1000  (cost=0.00..2180.75 rows=142602171 width=0)
>                                                   Index Cond:
>(rec_insert_time >= '2019-10-01 08:20:38.115471-05'::timestamp with time
>zone)
>
>
>Let me know if this rings any bells!  I will respond to other comments with
>other replies.
>

My guess - it's (at least partially) due to cpu_operator_cost,
associated with the now() call. When replaced with a literal, this cost
disappears and so the total query cost decreases.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services 

On Tue, 15 Oct 2019 at 08:43, Jeremy Finzel <finzelj@gmail.com> wrote:
> I wanted to follow up on this specific issue.  Isn't this the heart of the matter and a fundamental problem?  If I
wantto rely on BRIN indexes as in a straightforward case as explained in OP, but I don't know if the planner will be
nearlyreliable enough, how can I depend on them in production?  Is this not considered a planner bug or should this
kindof case be documented as problematic for BRIN?  As another way to look at it: is there a configuration parameter
thatcould be added specific to BRIN or bitmapscan to provide help to cases like this? 
>
> On freshly analyzed tables, I tried my original query again and found that even with now() - 3 days it does not
choosethe BRIN index.  In fact, it chose another btree on the table like (id1, id2, rec_insert_time).  With warm cache,
thepg-chosen plan takes 40 seconds to execute, whereas when I force a BRIN scan it takes only 4 seconds. 

Another thing which you might want to look at is the correlation
column in the pg_stats view for the rec_insert_time column. Previous
to 7e534adcd, BRIN index were costed based on the selectivity
estimate. There was no accountability towards the fact that the pages
for those records might have been spread out over the entire table.
Post 7e534adcd, we use the correlation estimate to attempt to estimate
how many pages (more specifically "ranges") we're likely to hit based
on that and the selectivity estimate. This commit intended to fix the
issue we had with BRIN indexes being selected far too often.  Of
course, the correlation is based on the entire table, if there are
subsets of the table that are perhaps perfectly correlated, then the
planner is not going to know about that. It's possible that some of
your older rec_insert_times are spread out far more than the newer
ones.  As a test, you could try creating a new table and copying the
records over to it in rec_insert_time order and seeing if the BRIN
index is selected for that table (after having performed an ANALYZE).

It would be interesting if you could show the pg_stats row for the
column so that we can see if the correlation is low.

You can see from the code below that the final selectivity strongly
influenced by the correlation value (REF: brincostestimate)

qualSelectivity = clauselist_selectivity(root, indexQuals,
baserel->relid,
JOIN_INNER, NULL);

/* work out the actual number of ranges in the index */
indexRanges = Max(ceil((double) baserel->pages / statsData.pagesPerRange),
  1.0);

/*
* Now calculate the minimum possible ranges we could match with if all of
* the rows were in the perfect order in the table's heap.
*/
minimalRanges = ceil(indexRanges * qualSelectivity);

/*
* Now estimate the number of ranges that we'll touch by using the
* indexCorrelation from the stats. Careful not to divide by zero (note
* we're using the absolute value of the correlation).
*/
if (*indexCorrelation < 1.0e-10)
estimatedRanges = indexRanges;
else
estimatedRanges = Min(minimalRanges / *indexCorrelation, indexRanges);

/* we expect to visit this portion of the table */
selec = estimatedRanges / indexRanges;

CLAMP_PROBABILITY(selec);


My overall view on this is that the BRIN index is not that great since
it's not eliminating that many rows by using it.

From above we see:

>  Bitmap Heap Scan on foo.log_table l  (cost=2391.71..24360848.29 rows=735 width=99) (actual time=824.133..21329.054
rows=466loops=1) 
                                       Output: <hidden>
                                       Recheck Cond:
(l.rec_insert_time >= (now() - '10 days'::interval))
                                       Rows Removed by Index Recheck: 8187584
                                       Filter: ((l.field1 IS NOT NULL)
AND (l.category = 'music'::name))
                                       Rows Removed by Filter: 19857107
                                       Heap Blocks: lossy=1509000

So you have just 466 rows matching these quals, but the executor had
to scan 1.5 million pages to get those and filter out 8.1 million rows
on the recheck then 19.8 million on the filter. You've mentioned that
the table's heap is 139 GB, which is about 18 million pages.  It seems
your query would perform much better if you had a btree index such as
(category, rec_insert_time) where field1 is not null;,

Of course, you've mentioned that you are finding when the plan uses
the BRIN index that it executes more quickly, but I think you're going
to find BRIN unreliable for tables anything other than INSERT-only
tables which the records are always inserted with an ever-increasing
or decreasing value in the BRIN indexed column.  If you start
performing UPDATEs then that's going to create holes that new record
will fill and cause the correlation to start dropping resulting in the
BRIN indexes scan cost going up.

On the other hand, if you think you can do better than what was done
in 7e534adcd, then it would be good to see someone working on it. I'm
sure something better can be done. It's just not that easy to do with
the scant correlation data we have on the column.

As for is this a bug or something that's missing from the documents.
The documents do mention:

"BRIN stands for Block Range Index. BRIN is designed for handling very
large tables in which certain columns have some natural correlation
with their physical location within the table."

https://www.postgresql.org/docs/current/brin-intro.html

--
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

This reminds me of an issue I reported several years ago where Btree index
scans were chosen over seq scan of a large, INSERT-only table due to very high
correlation, but performed poorly.  I concluded that use of the the high "large
scale" correlation on a large 50+GB table caused the planner to fail to account
for a larger number of pages being read nonsequentially (the opposite of your
issue).  I think that's because we were INSERTing data which was at least
approximately sorted on record END time, and the index was on record START
time.  For a large table with a week's data, the correlation of "start time"
was still be very high (0.99995).  But scanning the index ends up reading pages
nonsequentially, and also multiple visits per page.

I eeked out a patch which made "correlation" a per-index statistic rather than
a per-column one.  That means the planner could distinguish between a
freshly-built btree index and a fragmented one.  (At the time, there was a
hypothesis that our issue was partially due to repeated values of the index
columns.)  It didn't occur to me at the time, but that would also allow
creating numerous, partial BRIN indices, each of which would have separate
correlation computed over just their "restricted range", which *might* also
handle your case, depending on how packed your data is.

https://www.postgresql.org/message-id/flat/20170707234119.GN17566%40telsasoft.com#fdcbebc342b8fb9ad0ff293913f54d11

On Tue, Oct 15, 2019 at 11:05:13AM -0500, Jeremy Finzel wrote:
> I do believe that the only use case that will work really well for BRIN is
> either a truly insert-only table which is never pruned ...  or a table which
> is routinely CLUSTERed!

Or partitioned table, which for large data sets I highly recommend instead of
DELETE.

Justin

On Wed, 16 Oct 2019 at 05:05, Jeremy Finzel <finzelj@gmail.com> wrote:
> But perhaps it would be worth exploring if there could be more detailed stats on physical vs logical correlation,
suchas when ANALYZE takes its samples, noting physical locations as well as logical values, and allowing the
correlationto take account of that more detailed analysis.  Of course, sounds like a huge amount of work with uncertain
benefits.

Yes. I think improving the statistics could be beneficial. It does
appear like the single value for the entire column is not fine-grained
enough for your use case.

> As the docs state, I do believe that the only use case that will work really well for BRIN is either a truly
insert-onlytable which is never pruned (an idea I dislike as a DBA who wants us to keep OLTP data trim and implement
dataretention policies :), or a table which is routinely CLUSTERed! 

Have you considered partitioning the table so that the retention
policy could be implemented by dropping a partition rather than
performing a bulk DELETE?


--
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

On Wed, 16 Oct 2019 at 11:40, Justin Pryzby <pryzby@telsasoft.com> wrote:
> It didn't occur to me at the time, but that would also allow
> creating numerous, partial BRIN indices, each of which would have separate
> correlation computed over just their "restricted range", which *might* also
> handle your case, depending on how packed your data is.

Perhaps I've misunderstood you, but the correlation that's used is per
column, not per index. The only way to have it calculate multiple
correlations would be to partition the table. There'd be a correlation
for the column on each partition that way.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services