Re: Parallel Seq Scan

On Sat, Oct 17, 2015 at 11:45 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
>
> Agreed and on looking at code, I think in below code, if we pass
> parallelOK as true for the cases where Portal is non-null, such a
> problem could happen.
>
>
> static int
>
> exec_run_select(PLpgSQL_execstate *estate,
>
> PLpgSQL_expr *expr, long maxtuples, Portal *portalP,
>
> bool parallelOK)
>
> {
>
> ParamListInfo paramLI;
>
> int rc;
>
>
> /*
>
> * On the first call for this expression generate the plan
>
> */
>
> if (expr->plan == NULL)
>
> exec_prepare_plan(estate, expr, parallelOK ?
>
> CURSOR_OPT_PARALLEL_OK : 0);
>
>
> /*
>
> * If a portal was requested, put the query into the portal
>
> */
>
> if (portalP != NULL)
>
> {
>
> /*
>
> * Set up short-lived ParamListInfo
>
> */
>
> paramLI = setup_unshared_param_list(estate, expr);
>
>
> *portalP = SPI_cursor_open_with_paramlist(NULL, expr->plan,
>
> paramLI,
>
> estate->readonly_func);
>
>
>
>
> and one such case is
>
> exec_stmt_return_query()
> {
> ..
>
> if (stmt->query != NULL)
>
> {
>
> /* static query */
>
> exec_run_select(estate, stmt->query, 0, &portal, true);
>
> ..
> }
>
>
> In this function we are using controlled fetch mechanism (count as 50) to
> fetch the tuples which we initially thought of not supporting for
parallelism,
> as such a mechanism is not built for parallel workers and that is the
> reason we want to prohibit cases where ever cursor is getting created.
>

Here, I wanted to say that is one of the reasons for prohibiting cursors
for parallelism, certainly there are others like Backward scan.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com