Parallel query fails on standby server

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;
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 <http://www.enterprisedb.com>*