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(1,5000000));
edb=# analyze ert;
edb=# vacuum ert;

*Slave :-*

edb=# set max_parallel_degree =5;
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:

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
PFA patch to fix the issue.

With Regards,
Ashutosh Sharma
EnterpriseDB: *http://www.enterprisedb.com <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.
 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())
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to