Обсуждение: analyze causes query planner to choose suboptimal plan for a selectquery in separate transaction

On 4/25/19 7:37 AM, Martin Kováčik wrote:
> Hi group,
> 

See comments inline below

> To illustrate my situation let's consider my tests look like this:
> 
> BEGIN;
> 
> -- A: insert data for the test
> 
> -- B: i'll refer to this point later
> 
> -- C: select(s)
> 
> ROLLBACK;
> 
> Everything is fine, until autovacuum (analyze) runs when the test is at 
> point B. After that the query planner at point C chooses wrong plan and 
> the query takes a long time to complete, blocking one CPU core for a 
> long time. It seems like the planner statistics inside running 
> transaction are affected by analyze task running outside of the 
> transaction. In this case after running analyze (outside the 
> transaction) when the transaction is at B, causes query planner to think 
> there are no rows (because the inserts at point A were not yet committed).
> 
> I did prepare a simple test case to reproduce this behavior:
> 
> First you need to create a table:
> 
> create table a (id bigint primary key);
> 
> Then run this transaction:
> 
> begin;
> insert into a
> select * from generate_series(1, 1000);
> 
> -- during sleep execute analyze on this db in separate connection
> select pg_sleep(10);

analyze a;

On my machine that changes the time from:

29715.763 ms

to

291.765 ms

when running ANALYZE in the concurrent connection during the sleep.


> 
> explain analyze select count(*) from (
> select distinct a1, a2, a3, a4 from a a1
> left join a a2 on a1.id <http://a1.id> > 900
> left join a a3 on a2.id <http://a2.id> = a3.id <http://a3.id>
> left join a a4 on a3.id <http://a3.id> = a4.id <http://a4.id>
> left join a a5 on a5.id <http://a5.id> = a4.id <http://a4.id>
> left join a a6 on a6.id <http://a6.id> = a5.id <http://a5.id>
> left join a a7 on a7.id <http://a7.id> = a6.id <http://a6.id>
> left join a a8 on a8.id <http://a8.id> = a7.id <http://a7.id>) temp;
> 
> rollback;
> 



-- 
Adrian Klaver
adrian.klaver@aklaver.com

=?UTF-8?B?TWFydGluIEtvdsOhxI1paw==?= <kovacik@redbyte.eu> writes:
> To illustrate my situation let's consider my tests look like this:

> BEGIN;

> -- A: insert data for the test

> -- B: i'll refer to this point later

> -- C: select(s)

> ROLLBACK;

> Everything is fine, until autovacuum (analyze) runs when the test is at
> point B. After that the query planner at point C chooses wrong plan and the
> query takes a long time to complete, blocking one CPU core for a long time.
> It seems like the planner statistics inside running transaction are
> affected by analyze task running outside of the transaction.

Yup, they are.  However, you're already at risk of a pretty bad plan for
a case like this, since (by assumption) the stats before you did the
insert at step A are radically different from what they should be after
the insert.

The standard recommendation, when you need the results of a data change
to be understood by the planner immediately, is to do your own ANALYZE:

BEGIN;

-- A: insert data for the test

ANALYZE test_table;

-- C: select(s)

ROLLBACK;

This should protect step C against seeing any irrelevant stats, because

(a) once your transaction has done an ANALYZE, autovacuum shouldn't
think the stats are out of date, and

(b) even if it does, your transaction is now holding
ShareUpdateExclusiveLock on the table so auto-ANALYZE can't get
that lock to do a fresh ANALYZE.


Another thing you could do is not even allow the test table to exist
outside your transaction:

BEGIN;

CREATE TABLE test_table (...);

-- A: insert data for the test

ANALYZE test_table;  -- this is now somewhat optional

-- C: select(s)

ROLLBACK;

Then there's nothing for auto-ANALYZE to get its hands on.  If you're
satisfied with the planner's default behavior in the absence of any
stats, you could omit the post-insertion ANALYZE in this case.  But
I'm not sure that that would represent a test that has much to do with
production situations.

            regards, tom lane

On 4/25/19 12:47 PM, Martin Kováčik wrote:
> As my example shows you don't have to import a lot of rows - 1000 is 
> enough to make a difference - it all depends on the query. When a 
> cartesian product is involved only a few records is enough.
> I think that stats should be MVCC versioned otherwise the planner is 
> using wrong statistics and chooses wrong plans.

Then you are looking at moving the choke point to looking up the correct 
stats across possibly hundreds/thousands of transactions in flight.

> *Martin Kováčik*
> /CEO/
> *redByte*, s.r.o.
> +421 904 236 791
> kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>, www.redbyte.eu 
> <http://redbyte.eu>
> 
> 
> On Thu, Apr 25, 2019 at 9:28 PM Michael Lewis <mlewis@entrata.com 
> <mailto:mlewis@entrata.com>> wrote:
> 
> 
> 
>     On Thu, Apr 25, 2019, 11:34 AM Martin Kováčik <kovacik@redbyte.eu
>     <mailto:kovacik@redbyte.eu>> wrote:
> 
>         Turning off autovacuum for the tests is a valid option and I
>         will definitely do this as a workaround. Each test pretty much
>         starts with empty schema and data for it is generated during the
>         run and rolled back at the end. I have a lot of tests and at the
>         moment it is not feasible to modify them.
> 
>         The real workload for the application is different, but there
>         are some cases, when we import data from remote web service in a
>         transaction do some work with it and then we do a commit. If
>         there is an autovacuum during this process I assume there will
>         be similar problem regarding planner statistics.
> 
> 
>     Unless you are importing a huge amount of data relative to what is
>     already there, it seems likely to be significantly less impactful
>     than adding data to a completely empty table. The stats on a table
>     with 0 rows and then 5000 rows is going to be night and day, while
>     the difference between stats on 100,000 rows and 105,000 is not as
>     impactful. Musing here. I expect others will chime in.
> 
>     Stats are not versioned with MVCC so it would expected that a commit
>     in another transaction that is updating stats would influence the
>     query plan for another transaction that is active.
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 4/25/19 1:28 PM, Martin Kováčik wrote:
> I'm not sure if I understand what you mean. My initial thought was that 
> stats are fixed per transaction, i.e. analyze from comitted transaction 
> doesn't interfere with another running transaction. Maybe I was confused 
> by this because analyze can be run inside a transaction, so my 
> assumption was it is isolated like other statements.

To make it clearer I would take a look at:

https://www.postgresql.org/docs/11/sql-analyze.html
https://www.postgresql.org/docs/11/catalog-pg-statistic.html


The gist is currently there is one entry(with exception noted in second 
link) per column in each table. This is the most recent information 
available subject to the conditions Tom pointed out in his post. To make 
it MVCC aware like you suggest would mean tracking the state of all 
transactions currently open and recording that information, so each 
transaction could find its stats. This means the planning problem could 
then move to the stats table as it would need to be analyzed itself to 
work efficiently. This would probably also need to be MVCC aware to be 
relevant, which would add to the overhead. I could see this turn into a 
'hall of mirrors' problem quickly.

> *Martin Kováčik*
> /CEO/
> *redByte*, s.r.o.
> +421 904 236 791
> kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>, www.redbyte.eu 
> <http://redbyte.eu>
> 
> 
> On Thu, Apr 25, 2019 at 9:58 PM Adrian Klaver <adrian.klaver@aklaver.com 
> <mailto:adrian.klaver@aklaver.com>> wrote:
> 
>     On 4/25/19 12:47 PM, Martin Kováčik wrote:
>      > As my example shows you don't have to import a lot of rows - 1000 is
>      > enough to make a difference - it all depends on the query. When a
>      > cartesian product is involved only a few records is enough.
>      > I think that stats should be MVCC versioned otherwise the planner is
>      > using wrong statistics and chooses wrong plans.
> 
>     Then you are looking at moving the choke point to looking up the
>     correct
>     stats across possibly hundreds/thousands of transactions in flight.
> 
>      > *Martin Kováčik*
>      > /CEO/
>      > *redByte*, s.r.o.
>      > +421 904 236 791
>      > kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>
>     <mailto:kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>>,
>     www.redbyte.eu <http://www.redbyte.eu>
>      > <http://redbyte.eu>
>      >
>      >
>      > On Thu, Apr 25, 2019 at 9:28 PM Michael Lewis <mlewis@entrata.com
>     <mailto:mlewis@entrata.com>
>      > <mailto:mlewis@entrata.com <mailto:mlewis@entrata.com>>> wrote:
>      >
>      >
>      >
>      >     On Thu, Apr 25, 2019, 11:34 AM Martin Kováčik
>     <kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>
>      >     <mailto:kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>>> wrote:
>      >
>      >         Turning off autovacuum for the tests is a valid option and I
>      >         will definitely do this as a workaround. Each test pretty
>     much
>      >         starts with empty schema and data for it is generated
>     during the
>      >         run and rolled back at the end. I have a lot of tests and
>     at the
>      >         moment it is not feasible to modify them.
>      >
>      >         The real workload for the application is different, but there
>      >         are some cases, when we import data from remote web
>     service in a
>      >         transaction do some work with it and then we do a commit. If
>      >         there is an autovacuum during this process I assume there
>     will
>      >         be similar problem regarding planner statistics.
>      >
>      >
>      >     Unless you are importing a huge amount of data relative to
>     what is
>      >     already there, it seems likely to be significantly less impactful
>      >     than adding data to a completely empty table. The stats on a
>     table
>      >     with 0 rows and then 5000 rows is going to be night and day,
>     while
>      >     the difference between stats on 100,000 rows and 105,000 is
>     not as
>      >     impactful. Musing here. I expect others will chime in.
>      >
>      >     Stats are not versioned with MVCC so it would expected that a
>     commit
>      >     in another transaction that is updating stats would influence the
>      >     query plan for another transaction that is active.
>      >
> 
> 
>     -- 
>     Adrian Klaver
>     adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

I’ve got it now, thank you for the clarification. You are right.

Martin Kováčik
+421904236791

> On 26 Apr 2019, at 00:25, Adrian Klaver <adrian.klaver@aklaver.com> wrote:
>
>> On 4/25/19 1:28 PM, Martin Kováčik wrote:
>> I'm not sure if I understand what you mean. My initial thought was that stats are fixed per transaction, i.e.
analyzefrom comitted transaction doesn't interfere with another running transaction. Maybe I was confused by this
becauseanalyze can be run inside a transaction, so my assumption was it is isolated like other statements. 
>
> To make it clearer I would take a look at:
>
> https://www.postgresql.org/docs/11/sql-analyze.html
> https://www.postgresql.org/docs/11/catalog-pg-statistic.html
>
>
> The gist is currently there is one entry(with exception noted in second link) per column in each table. This is the
mostrecent information available subject to the conditions Tom pointed out in his post. To make it MVCC aware like you
suggestwould mean tracking the state of all transactions currently open and recording that information, so each
transactioncould find its stats. This means the planning problem could then move to the stats table as it would need to
beanalyzed itself to work efficiently. This would probably also need to be MVCC aware to be relevant, which would add
tothe overhead. I could see this turn into a 'hall of mirrors' problem quickly. 
>
>> *Martin Kováčik*
>> /CEO/
>> *redByte*, s.r.o.
>> +421 904 236 791
>> kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>, www.redbyte.eu <http://redbyte.eu>
>> On Thu, Apr 25, 2019 at 9:58 PM Adrian Klaver <adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>> wrote:
>>    On 4/25/19 12:47 PM, Martin Kováčik wrote:
>>     > As my example shows you don't have to import a lot of rows - 1000 is
>>     > enough to make a difference - it all depends on the query. When a
>>     > cartesian product is involved only a few records is enough.
>>     > I think that stats should be MVCC versioned otherwise the planner is
>>     > using wrong statistics and chooses wrong plans.
>>    Then you are looking at moving the choke point to looking up the
>>    correct
>>    stats across possibly hundreds/thousands of transactions in flight.
>>     > *Martin Kováčik*
>>     > /CEO/
>>     > *redByte*, s.r.o.
>>     > +421 904 236 791
>>     > kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>
>>    <mailto:kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>>,
>>    www.redbyte.eu <http://www.redbyte.eu>
>>     > <http://redbyte.eu>
>>     >
>>     >
>>     > On Thu, Apr 25, 2019 at 9:28 PM Michael Lewis <mlewis@entrata.com
>>    <mailto:mlewis@entrata.com>
>>     > <mailto:mlewis@entrata.com <mailto:mlewis@entrata.com>>> wrote:
>>     >
>>     >
>>     >
>>     >     On Thu, Apr 25, 2019, 11:34 AM Martin Kováčik
>>    <kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>
>>     >     <mailto:kovacik@redbyte.eu <mailto:kovacik@redbyte.eu>>> wrote:
>>     >
>>     >         Turning off autovacuum for the tests is a valid option and I
>>     >         will definitely do this as a workaround. Each test pretty
>>    much
>>     >         starts with empty schema and data for it is generated
>>    during the
>>     >         run and rolled back at the end. I have a lot of tests and
>>    at the
>>     >         moment it is not feasible to modify them.
>>     >
>>     >         The real workload for the application is different, but there
>>     >         are some cases, when we import data from remote web
>>    service in a
>>     >         transaction do some work with it and then we do a commit. If
>>     >         there is an autovacuum during this process I assume there
>>    will
>>     >         be similar problem regarding planner statistics.
>>     >
>>     >
>>     >     Unless you are importing a huge amount of data relative to
>>    what is
>>     >     already there, it seems likely to be significantly less impactful
>>     >     than adding data to a completely empty table. The stats on a
>>    table
>>     >     with 0 rows and then 5000 rows is going to be night and day,
>>    while
>>     >     the difference between stats on 100,000 rows and 105,000 is
>>    not as
>>     >     impactful. Musing here. I expect others will chime in.
>>     >
>>     >     Stats are not versioned with MVCC so it would expected that a
>>    commit
>>     >     in another transaction that is updating stats would influence the
>>     >     query plan for another transaction that is active.
>>     >
>>    --     Adrian Klaver
>>    adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
>
>
> --
> Adrian Klaver
> adrian.klaver@aklaver.com