Re: Pluggable Storage - Andres's take

From: Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Pluggable Storage - Andres's take
Date: 2018-10-23 06:49:23
Message-ID: CAJrrPGf2pAv+ZqYXVwgULAKs5U18WB5ZKw3QSL1zy=BntQZCEQ@mail.gmail.com
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On Mon, Oct 22, 2018 at 6:16 PM Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
wrote:

> On Thu, Oct 18, 2018 at 1:04 PM Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
> wrote:
>
>> On Tue, Oct 9, 2018 at 1:46 PM Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
>> wrote:
>>
>>>
>>> I also observed the failure of aggregates.sql, will look into it.
>>>
>>
>> The random failure of aggregates.sql is as follows
>>
>> SELECT avg(a) AS avg_32 FROM aggtest WHERE a < 100;
>> ! avg_32
>> ! ---------------------
>> ! 32.6666666666666667
>> (1 row)
>>
>> -- In 7.1, avg(float4) is computed using float8 arithmetic.
>> --- 8,16 ----
>> (1 row)
>>
>> SELECT avg(a) AS avg_32 FROM aggtest WHERE a < 100;
>> ! avg_32
>> ! --------
>> !
>> (1 row)
>>
>> Same NULL result for another aggregate query on column b.
>>
>> The aggtest table is accessed by two tests that are running in parallel.
>> i.e aggregates.sql and transactions.sql, In transactions.sql, inside a
>> transaction
>> all the records in the aggtest table are deleted and aborted the
>> transaction,
>> I suspect that some visibility checks are having some race conditions
>> that leads
>> to no records on the table aggtest, thus it returns NULL result.
>>
>> If I try the scenario manually by opening a transaction and deleting the
>> records, the
>> issue is not occurring.
>>
>> I am yet to find the cause for this problem.
>>
>
> I am not yet able to generate a test case where the above issue can occur
> easily for
> debugging, it is happening randomly. I will try to add some logs to find
> out the problem.
>

I am able to generate the simple test and found the problem. The issue with
the following
SQL.

SELECT *
INTO TABLE xacttest
FROM aggtest;

During the processing of the above query, the tuple that is selected from
the aggtest is
sent to the intorel_receive() function, and the same tuple is used for the
insert, because
of this reason, the tuple xmin is updated and it leads to failure of
selecting the data from
another query. I fixed this issue by materializing the slot.

During the above test run, I found another issue during analyze, that is
trying to access
the invalid offset data. Attached a fix patch.

Regards,
Haribabu Kommi
Fujitsu Australia

Attachment Content-Type Size
0001-scan-start-offset-fix-during-analyze.patch application/octet-stream 1.3 KB
0002-Materialize-all-the-slots-before-they-are-processed-.patch application/octet-stream 786 bytes

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