Re: TODO : Allow parallel cores to be used by vacuumdb [ WIP ]

On December 2014 17:31 Amit Kapila Wrote,

>I suggest rather than removing, edit the comment to indicate
>the idea behind code at that place.
Done

>Okay, I think this part of code is somewhat similar to what
>is done in pg_dump/parallel.c with some differences related
>to handling of inAbort. One thing I have noticed here which
>could lead to a problem is that caller of select_loop() function
>assumes that return value is less than zero only if there is a
>cancel request which I think is wrong, because select system
>call could also return -1 in case of error. I am referring below
>code in above context:
+ if (i < 0)
+ {
+ /*
+ * This can only happen if user has sent the cancel request using
+ * Ctrl+C, Cancel is handled by 0th slot, so fetch the error result.
+ */
+
+ GetQueryResult(pSlot[0].connection, dbname, progname,
+ completedb);

Now for abort case I am using special error code, and other than that case we will assert, this behavior is same as in pg_dump.

>Hmm, theoretically I think new behaviour could lead to more I/O in
>certain cases as compare to existing behaviour. The reason for more I/O
>is that in the new behaviour, while doing Analyze for a particular table at
>different targets, in-between it has Analyze of different table as well,
>so the pages in shared buffers or OS cache for a particular table needs to
>be reloded again for a new target whereas currently it will do all stages
>of Analyze for a particular table in one-go which means that each stage
>of Analyze could get benefit from the pages of a table loaded by previous
>stage. If you agree, then we should try to avoid this change in new
>behaviour.

I will work on this comment and post the updated patch..

I will move this patch to the latest commitfest.

Regards,
Dilip