Re: [HACKERS] Parallel query fails on standby server

[Michael Paquier]

On Wed, Mar 9, 2016 at 12:34 AM, Robert Haas  wrote:
> On Tue, Mar 8, 2016 at 8:23 AM, Michael Paquier
>  wrote:
>> On Tue, Mar 8, 2016 at 9:51 PM, Craig Ringer  wrote:
>>> On 8 March 2016 at 20:30, Ashutosh Sharma  wrote:

 While testing a parallel scan feature on standby server, it is found that
 the parallel query fails with an error "ERROR:  failed to initialize
 transaction_read_only to 0".

>>>
>>> Looks like it might be a good idea to add some tests to src/test/recovery
>>> for parallel query on standby servers...
>>
>> An even better thing would be a set of read-only tests based on the
>> database "regression" generated by make check, itself run with
>> pg_regress.
>
> I'm not sure anything in the main regression suite actually goes
> parallel right now, which is probably the first thing to fix.
>
> Unless, of course, you use force_parallel_mode=regress, max_parallel_degree>0.

I was thinking about a test in src/test/recovery, that runs a standby
and a master. pg_regress with the main recovery test suite is run on
the master, then a second pg_regress run happens with a set of
read-only queries, set with sql/expected located in src/test/recovery
directly for example. Do we actually have a buildfarm animal using
those parameters in extra_config?
-- 
Michael


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Re: [HACKERS] Parallel query fails on standby server

[Robert Haas]

On Tue, Mar 8, 2016 at 8:23 AM, Michael Paquier
 wrote:
> On Tue, Mar 8, 2016 at 9:51 PM, Craig Ringer  wrote:
>> On 8 March 2016 at 20:30, Ashutosh Sharma  wrote:
>>>
>>> While testing a parallel scan feature on standby server, it is found that
>>> the parallel query fails with an error "ERROR:  failed to initialize
>>> transaction_read_only to 0".
>>>
>>
>> Looks like it might be a good idea to add some tests to src/test/recovery
>> for parallel query on standby servers...
>
> An even better thing would be a set of read-only tests based on the
> database "regression" generated by make check, itself run with
> pg_regress.

I'm not sure anything in the main regression suite actually goes
parallel right now, which is probably the first thing to fix.

Unless, of course, you use force_parallel_mode=regress, max_parallel_degree>0.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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Re: [HACKERS] Parallel query fails on standby server

[Michael Paquier]

On Tue, Mar 8, 2016 at 9:51 PM, Craig Ringer  wrote:
> On 8 March 2016 at 20:30, Ashutosh Sharma  wrote:
>>
>>
>> While testing a parallel scan feature on standby server, it is found that
>> the parallel query fails with an error "ERROR:  failed to initialize
>> transaction_read_only to 0".
>>
>
> Looks like it might be a good idea to add some tests to src/test/recovery
> for parallel query on standby servers...

An even better thing would be a set of read-only tests based on the
database "regression" generated by make check, itself run with
pg_regress.
-- 
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Re: [HACKERS] Parallel query fails on standby server

[Craig Ringer]

On 8 March 2016 at 20:30, Ashutosh Sharma  wrote:

>
> While testing a parallel scan feature on standby server, it is found that
> the parallel query fails with an error "*ERROR:  failed to initialize
> transaction_read_only to 0*".
>
>
Looks like it might be a good idea to add some tests to src/test/recovery
for parallel query on standby servers...

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

[HACKERS] Parallel query fails on standby server

[Ashutosh Sharma]

Hi All,

While testing a parallel scan feature on standby server, it is found that
the parallel query fails with an error "*ERROR:  failed to initialize
transaction_read_only to 0*".

Following are the steps used to reproduce the issue:

*Master :-*

edb=# create table ert(n int);
edb=# insert into  ert values (generate_series(1,500));
edb=# analyze ert;
edb=# vacuum ert;

*Slave :-*

edb=# set max_parallel_degree =5;
SET
edb=# explain analyze verbose  select * from ert where n<=1000;
ERROR:  failed to initialize transaction_read_only to 0
CONTEXT:  parallel worker, PID 26042

*Root cause Analysis:*  After debugging the worker, it is observed that in
*RestoreGUCState()*, if a guc var can't be skipped it is Initialiazed with
a default value and
in this process when a guc variable "*transaction_read_only*" is being
Initialzed it calls a check_hook *check_transaction_read_only()* which
eventually fails due to
below check which says the guc var "*transaction_read_only*" can't be set
while recovery is in progress:






*if
(RecoveryInProgress()){GUC_check_errcode(ERRCODE_FEATURE_NOT_SUPPORTED);GUC_check_errmsg("cannot
set transaction read-write mode during recovery");return false;}*

*Solution:* Make use of a global variable "*InitializingParallelWorker"* to
protect the check for *RecoveryInProgress()* when Parallel Worker is being
Initialsed.
PFA patch to fix the issue.

With Regards,
Ashutosh Sharma
EnterpriseDB: *http://www.enterprisedb.com *
diff --git a/src/backend/commands/variable.c b/src/backend/commands/variable.c
index 903b3a6..c7173c9 100644
--- a/src/backend/commands/variable.c
+++ b/src/backend/commands/variable.c
@@ -482,11 +482,13 @@ show_log_timezone(void)
  * nothing since XactReadOnly will be reset by the next StartTransaction().
  * The IsTransactionState() test protects us against trying to check
  * RecoveryInProgress() in contexts where shared memory is not accessible.
+ * We can also skip the check for RecoveryInProgress while initializing the
+ * Parallel Workers by making use of the global variable InitializingParallelWorker.
  */
 bool
 check_transaction_read_only(bool *newval, void **extra, GucSource source)
 {
-	if (*newval == false && XactReadOnly && IsTransactionState())
+	if (*newval == false && XactReadOnly && IsTransactionState() && !InitializingParallelWorker)
 	{
 		/* Can't go to r/w mode inside a r/o transaction */
 		if (IsSubTransaction())

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