[HACKERS] Enabling parallelism for queries coming from SQL or other PL functions

Hello everybody,

In the current version, queries in SQL or PL functions can not
leverage parallelism. To relax this restriction I prepared a patch,
the approach used in the patch is explained next,

Approach:

1. Allow parallelism for queries in PL functions by passing
CURSOR_OPT_PARALLEL_OK instead of 0 to exec_prepare_plan called from
exec_stmt_execsql or exec_stmt_dynexecute. Similarly, pass
CURSOR_OPT_PARALLEL_OK instead of 0 to SPI_execute and exec_run_select
called from exec_stmt_dynexecute. CURSOR_OPT_PARALLEL_OK is passed to
these functions after checking if the statement is not trigger, since
in that case using parallelism may not be efficient.

2. In ExecutePlan there is an additional check to see if the query is
coming from SQL or PL functions and is having a parallel plan. In that
case we ignore the check of numberTuples since it is always one for
these functions and existing checks restrict parallelism for these
cases. Though, I understand this may not be the most elegant way to do
this, and would be pleased to know some better alternative.


I have attached a sql file containing cases for some pgpsql, perl,
python functions and an .out file which contains the parallel plans
for the queries in these functions after this patch. This might be
helpful in understanding the level of parallelism this patch is
relaxing for PL functions.

Thanks to my colleagues Amit Kapila and Dilip Kumar for discussions in
this regard.

-- 
Regards,
Rafia Sabih
EnterpriseDB: http://www.enterprisedb.com/

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