[Postgres-xc-developers] XC Stuck in a Loop

[Matt W. <MW...@XI...>]

Short version: it looks like there's a problem with ExecNestLoop and ExecProcNode recursively calling each other and getting stuck in a loop that never completes.

Details:
I've been experimenting with XC using fairly large tables. In this case it's 4 tables, 2 replicated, 2 distributed by hash. The select statement is a mere 31 lines long and contains a group by on 2 columns of one of the tables. The query never completes, even days later.

I'm using dtrace and a "git clone" version of XC from a few days ago compiled with the debug flag (-g). I see that ExecNestLoop and ExecProcNode appear to be calling each other heavily, as in thousands of times per second. That is, I am seeing stack traces where one calls the other, but also vice versa.

In researching further, I see a note in execRemote.c that seems to indicate that recursively calling ExecProcNode is happening by design:

        /*
         * The current implementation of DMLs with RETURNING when run on replicated
         * tables returns row from one of the datanodes. In order to achieve this
         * ExecProcNode is repeatedly called saving one tuple and rejecting the rest.
         * Do we have a DML on replicated table with RETURNING?
         */

I don't know about the accuracy of the debugging and certainly I'm out of my element when poring through the XC source code, so my guess as to the source of the problem should be questioned.

What additional debugging information can I provide to assist with the correct identification and debugging of this problem?

Regards,

Matt

NOTICE OF CONFIDENTIALITY - This material is intended for the use of the individual or entity to which it is addressed, and may contain information that is privileged, confidential and exempt from disclosure under applicable laws.  BE FURTHER ADVISED THAT THIS EMAIL MAY CONTAIN PROTECTED HEALTH INFORMATION (PHI). BY ACCEPTING THIS MESSAGE, YOU ACKNOWLEDGE THE FOREGOING, AND AGREE AS FOLLOWS: YOU AGREE TO NOT DISCLOSE TO ANY THIRD PARTY ANY PHI CONTAINED HEREIN, EXCEPT AS EXPRESSLY PERMITTED AND ONLY TO THE EXTENT NECESSARY TO PERFORM YOUR OBLIGATIONS RELATING TO THE RECEIPT OF THIS MESSAGE.  If the reader of this email (and attachments) is not the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. Please notify the sender of the error and delete the e-mail you received. Thank you.