> On Jul 3, 2017, at 10:25 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Mon, Jul 3, 2017 at 8:57 PM, Simon Riggs <si...@2ndquadrant.com> wrote: >> On 30 June 2017 at 05:14, Amit Kapila <amit.kapil...@gmail.com> wrote: >> >>> This is explained in section 15.2 [1], refer below para: >>> "The query might be suspended during execution. In any situation in >>> which the system thinks that partial or incremental execution might >>> occur, no parallel plan is generated. For example, a cursor created >>> using DECLARE CURSOR will never use a parallel plan. Similarly, a >>> PL/pgSQL loop of the form FOR x IN query LOOP .. END LOOP will never >>> use a parallel plan, because the parallel query system is unable to >>> verify that the code in the loop is safe to execute while parallel >>> query is active." >> >> Can you explain "unable to verify that the code in the loop is safe to >> execute while parallel query is active". Surely we aren't pushing code >> in the loop into the actual query, so the safety of command in the FOR >> loop has nothing to do with the parallel safety of the query. >> >> Please give an example of something that would be unsafe? Is that >> documented anywhere, README etc? >> >> FOR x IN query LOOP .. END LOOP >> seems like a case that would be just fine, since we're going to loop >> thru every row or break early. >> > > It is not fine because we don't support partial execution support. In > above case, if the loop breaks, we can't break parallel query > execution. Now, I don't think it will impossible to support the same, > but as of now, parallel subsystem doesn't have such a support.
I can understand this, but wonder if I could use something like FOR I TOTALLY PROMISE TO USE ALL ROWS rec IN EXECUTE sql LOOP ... END LOOP; if I hacked the grammar up a bit. Would the problem go away, or would I still have problems when exceptions beyond my control get thrown inside the loop? And if exception handling is a problem in the loop, are exceptions somehow not a problem in other parallel queries? Obviously I mean that syntax above in jest, but the question is in ernest. Thanks, mark -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers