[PERFORM] Index only scan sometimes switches to sequential scan for small amount of rows
Hi,
Situation:
We have a table with 3,500,000+ rows, which contain items that need to
be printed or have been printed previously.
Most of these records have a status of 'PRINTED', we have a partial
index on this table WHERE status <> 'PRINTED'.
During normal operation there will be < 10 records matching 'NOT_YET_PRINTED'.
When using the index scan this is done in < 5ms, but when the
sequential scan is involved the query runs > 500ms.
We query this table often in the form:
SELECT *
FROM print_list
JOIN [...]
JOIN [...]
WHERE stats = 'NOT_YET_PRINTED'
LIMIT 8;
This query is currently switching between a sequential scan on the
print_list table and an index scan on the previously mentioned index.
When doing an explain analyze on the queries we see that it sometimes
expects to return > 5000 records when in reality it is only < 5
records that are returned, for example:
-> Index Scan using print_list_status_idx on print_list
(cost=0.27..1138.53 rows=6073 width=56) (actual time=0.727..0.727
rows=0 loops=1)
Sometimes, this results in the planner choosing a sequential scan for
this query.
When analyzing pg_stats we have sometimes have the following: (Note:
'NOT_YET_PRINTED' has not been found during this analyze, these are
real values)
attname| status
inherited | f
null_frac | 0
avg_width | 4
n_distinct | 3
most_common_vals | {PRINTED}
most_common_freqs | {0.996567}
histogram_bounds | {PREPARED,ERROR}
correlation| 0.980644
A question about this specific entry, which some of you may be able to
shed some light on:
most_common_vals contains only 1 entry, why is this? I would expect to
see 3 entries, as it has n_distinct=3
When looking at
http://www.postgresql.org/docs/current/static/row-estimation-examples.html
we can see that an estimate > 5000 is what is to be expected for these
statistics:
# select ( (1 - 0.996567)/2 * 350 )::int;
int4
--
6008
(1 row)
But why does it not record the frequency of 'PREPARED' and 'ERROR' in
most_common_*?
Our current strategies in mitigating this problem is decreasing the
autovacuum_*_scale_factor for this specific table, therefore
triggering more analyses and vacuums.
This is helping somewhat, as if the problem occurs it often solved
automatically if autoanalyze analyzes this table, it is analyzed many
times an hour currently.
We can also increase the 'Stats target' for this table, which will
cause the statistics to contain information about 'NOT_YET_PRINTED'
more often, but even then, it may not find any of these records, as
they sometimes do not exist.
Could you help us to find a strategy to troubleshoot this issue further?
Some specific questions:
- We can see it is doing a sequential scan of the full table (3.5mio
records) even when it only expects 8000 records to be returned, we
would expect this not to happen so soon.
- Why is most_common_* not filled when there are only 3 distinct values?
Feike Steenbergen
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Re: [PERFORM] Index only scan sometimes switches to sequential scan for small amount of rows
On 25.3.2015 13:04, Feike Steenbergen wrote:
...
> When analyzing pg_stats we have sometimes have the following: (Note:
> 'NOT_YET_PRINTED' has not been found during this analyze, these are
> real values)
>
> attname| status
> inherited | f
> null_frac | 0
> avg_width | 4
> n_distinct | 3
> most_common_vals | {PRINTED}
> most_common_freqs | {0.996567}
> histogram_bounds | {PREPARED,ERROR}
> correlation| 0.980644
>
> A question about this specific entry, which some of you may be able to
> shed some light on:
>
> most_common_vals contains only 1 entry, why is this? I would expect to
> see 3 entries, as it has n_distinct=3
To be included in the MCV list, the value has to actually appear in the
random sample at least twice, IIRC. If the values are very rare (e.g. if
you only have such 10 rows out of 3.5M), that may not happen.
You may try increasing the statistics target for this column, which
should make the sample larger and stats more detailed (max is 1,
which should use sample ~3M rows, i.e. almost the whole table).
> When looking at
> http://www.postgresql.org/docs/current/static/row-estimation-examples.html
> we can see that an estimate > 5000 is what is to be expected for these
> statistics:
>
> # select ( (1 - 0.996567)/2 * 350 )::int;
> int4
> --
> 6008
> (1 row)
>
> But why does it not record the frequency of 'PREPARED' and 'ERROR'
> in most_common_*?
Can you post results for this query?
SELECT stats, COUNT(*) FROM print_list group by 1
I'd like to know how frequent the other values are.
>
> Our current strategies in mitigating this problem is decreasing the
> autovacuum_*_scale_factor for this specific table, therefore
> triggering more analyses and vacuums.
I'm not sure this is a good solution. The problem is elsewhere, IMHO.
> This is helping somewhat, as if the problem occurs it often solved
> automatically if autoanalyze analyzes this table, it is analyzed
> many times an hour currently.
>
> We can also increase the 'Stats target' for this table, which will
> cause the statistics to contain information about 'NOT_YET_PRINTED'
> more often, but even then, it may not find any of these records, as
> they sometimes do not exist.
This is a better solution, IMHO.
>
> Could you help us to find a strategy to troubleshoot this issue
> further?
You might also make the index scans cheaper, so that the switch to
sequential scan happens later (when more rows are estimated). Try to
decreasing random_page_cost from 4 (default) to 1.5 or something like that.
It may hurt other queries, though, depending on the dataset size etc.
--
Tomas Vondrahttp://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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Re: [PERFORM] Index only scan sometimes switches to sequential scan for small amount of rows
Hi, thanks for having a look and thinking with us On 25 March 2015 at 13:45, Tomas Vondra wrote: > Can you post results for this query? > > SELECT stats, COUNT(*) FROM print_list group by 1 status | count +- ERROR | 159 PREPARED | 10162 PRINTED| 3551367 TO_BE_PREPARED | 2 (4 rows) >> We can also increase the 'Stats target' for this table, which will >> cause the statistics to contain information about 'NOT_YET_PRINTED' >> more often, but even then, it may not find any of these records, as >> they sometimes do not exist. > > This is a better solution, IMHO. We'll have a go at this, also if what you say about values having to appear at least twice, the other values may make it into most_common_*, which would make it clearer to us. We're a bit hesitant to decrease random_page_cost (currently 3 in this cluster) as a lot more is happening on this database. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index only scan sometimes switches to sequential scan for small amount of rows
I'm posting this as I am trying to understand what has happened.
TLDR: The problem seems to be fixed now.
By bumping the statistics_target we see that most_common_vals is
having its contents filled more often, causing way better estimates:
attname| status
inherited | f
null_frac | 0
avg_width | 4
n_distinct | 3
most_common_vals | {PRINTED,PREPARED,ERROR}
most_common_freqs | {0.996863,0.00307333,6.3e-05}
histogram_bounds | (null)
correlation| 0.98207
most_common_elems | (null)
most_common_elem_freqs | (null)
elem_count_histogram | (null)
Basically 100% of the records are accounted for in these statistics,
the planner now consistently estimates the number of rows to be very
small for other values.
Before bumping the target we didn't have information for 0.34% of the
rows, which in this case means roughly 11K rows.
What is the reasoning behind having at least 2 hits before including
it in the most_common_* columns?
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Re: [PERFORM] Index only scan sometimes switches to sequential scan for small amount of rows
Feike Steenbergen writes: > On 25 March 2015 at 13:45, Tomas Vondra wrote: >>> We can also increase the 'Stats target' for this table, which will >>> cause the statistics to contain information about 'NOT_YET_PRINTED' >>> more often, but even then, it may not find any of these records, as >>> they sometimes do not exist. >> This is a better solution, IMHO. > We'll have a go at this, also if what you say about values having to > appear at least twice, the other values may make it into > most_common_*, which would make it clearer to us. In principle increasing the stats target should fix this, whether or not 'NOT_YET_PRINTED' appears in the MCV list after any particular analyze; because what will happen is that the frequency for 'PRINTED' will more nearly approach 1, and so the estimated selectivity for other values will drop even if they're not in the list. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index only scan sometimes switches to sequential scan for small amount of rows
On Wed, Mar 25, 2015 at 9:07 AM, Feike Steenbergen <
feikesteenber...@gmail.com> wrote:
> I'm posting this as I am trying to understand what has happened.
> TLDR: The problem seems to be fixed now.
>
> By bumping the statistics_target we see that most_common_vals is
> having its contents filled more often, causing way better estimates:
>
> attname| status
> inherited | f
> null_frac | 0
> avg_width | 4
> n_distinct | 3
> most_common_vals | {PRINTED,PREPARED,ERROR}
> most_common_freqs | {0.996863,0.00307333,6.3e-05}
> histogram_bounds | (null)
> correlation| 0.98207
> most_common_elems | (null)
> most_common_elem_freqs | (null)
> elem_count_histogram | (null)
>
> Basically 100% of the records are accounted for in these statistics,
> the planner now consistently estimates the number of rows to be very
> small for other values.
>
> Before bumping the target we didn't have information for 0.34% of the
> rows, which in this case means roughly 11K rows.
>
> What is the reasoning behind having at least 2 hits before including
> it in the most_common_* columns?
>
If you sample a small portion of the table, then anything only present once
is going to be have a huge uncertainty on its estimate.
Consider the consequences of including things sampled once. 100% of the
rows that got sampled will be in the sample at least once. That means
most_common_freqs will always sum to 100%. Which means we are declaring
that anything not observed in the sample has a frequency of
0.%, which is clearly beyond what we have any reasonable
evidence to support.
Also, I doubt that that is the problem in the first place. If you collect
a sample of 30,000 (which the default target size of 100 does), and the
frequency of the second most common is really 0.00307333 at the time you
sampled it, you would expect to find it 92 times in the sample. The chances
against actually finding 1 instead of around 92 due to sampling error are
astronomical.
The problem seems to be rapidly changing stats, not too small of a target
size (unless your original target size was way below the current default
value, forgive me if you already reported that, I didn't see it anywhere).
If you analyze the table at a point when it is 100% PRINTED, there is no
way of knowing based on that analysis alone what the distribution of
!='PRINTED' would be, should such values ever arise.
Maybe it would work better if you built the partial index where status =
'NOT_YET_PRINTED', instead of !='PRINTED'.
Cheers,
Jeff
Re: [PERFORM] Index only scan sometimes switches to sequential scan for small amount of rows
On 25 March 2015 at 19:07, Jeff Janes wrote: > Also, I doubt that that is the problem in the first place. If you collect a > sample of 30,000 (which the default target size of 100 does), and the > frequency of the second most common is really 0.00307333 at the time you > sampled it, you would expect to find it 92 times in the sample. The chances > against actually finding 1 instead of around 92 due to sampling error are > astronomical. It can be that the distribution of values is very volatile; we hope the increased stats target (from the default=100 to 1000 for this column) and frequent autovacuum and autoanalyze helps in keeping the estimates correct. It seems that it did find some other records (<> 'PRINTED), as is demonstrated in the stats where there was only one value in the MCV list: the frequency was 0.996567 and the fraction of nulls was 0, therefore leaving 0.03+ for other values. But because none of them were in the MCV and MCF list, they were all treated as equals. They are certainly not equal. I not know why some values were found (they are mentioned in the histogram_bounds), but are not part of the MCV list, as you say, the likeliness of only 1 item being found is very small. Does anyone know the criteria for a value to be included in the MCV list? > The problem seems to be rapidly changing stats, not too small of a target > size (unless your original target size was way below the current default > value, forgive me if you already reported that, I didn't see it anywhere). > Maybe it would work better if you built the partial index where status = > 'NOT_YET_PRINTED', instead of !='PRINTED'. Thanks, we did create a partial index on 'NOT_YET_PRINTED' today to help aiding these kind of queries. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index only scan sometimes switches to sequential scan for small amount of rows
On Wed, Mar 25, 2015 at 1:00 PM, Feike Steenbergen < feikesteenber...@gmail.com> wrote: > On 25 March 2015 at 19:07, Jeff Janes wrote: > > > Also, I doubt that that is the problem in the first place. If you > collect a > > sample of 30,000 (which the default target size of 100 does), and the > > frequency of the second most common is really 0.00307333 at the time you > > sampled it, you would expect to find it 92 times in the sample. The > chances > > against actually finding 1 instead of around 92 due to sampling error are > > astronomical. > > It can be that the distribution of values is very volatile; we hope > the increased stats target (from the default=100 to 1000 for this > column) and frequent autovacuum and autoanalyze helps in keeping the > estimates correct. > > It seems that it did find some other records (<> 'PRINTED), as is > demonstrated in the stats where there was only one value in the MCV > list: the frequency was 0.996567 and the fraction of nulls was 0, > therefore leaving 0.03+ for other values. But because none of them > were in the MCV and MCF list, they were all treated as equals. They > are certainly not equal. > Now that I look back at the first post you made, it certainly looks like the statistics target was set to 1 when that was analyzed, not to 100. But it doesn't look quite correct for that, either. What version of PostgreSQL are running? 'select version();' What do you get when to do "analyze verbose print_list"? How can the avg_width be 4 when the vast majority of entries are 7 characters long? Cheers, Jeff
