Re: postgres_fdw join pushdown (was Re: Custom/Foreign-Join-APIs)

Thanks for the comments.

2015/05/01 22:35、Robert Haas <robertmhaas(at)gmail(dot)com> のメール：
> On Mon, Apr 27, 2015 at 5:07 AM, Shigeru Hanada
> <shigeru(dot)hanada(at)gmail(dot)com> wrote:
>> 2015-04-27 11:00 GMT+09:00 Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com>:
>>> Hanada-san, could you adjust your postgres_fdw patch according to
>>> the above new (previous?) definition.
>>
>> The attached v14 patch is the revised version for your v13 patch. It also contains changed for Ashutosh’s comments.
>
> We should probably move this discussion to a new thread now that the
> other patch is committed. Changing subject line accordingly.
>
> Generally, there's an awful lot of changes in this patch - it is over
> 2000 insertions and more than 450 deletions - and it's not awfully
> obvious why all of those changes are there. I think this patch needs
> a detailed README to accompany it explaining what the various changes
> in the patch are and why those things got changed; or maybe there is a
> way to break it up into multiple patches so that we can take a more
> incremental approach. I am really suspicious of the amount of
> wholesale reorganization of code that this patch is doing. It's
> really hard to validate that a reorganization like that is necessary,
> or that it's correct, and it's gonna make back-patching noticeably
> harder in the future. If we really need this much code churn it needs
> careful justification; if we don't, we shouldn't do it.
>

I agree. I’ll write detailed description for the patch and repost the new one with rebasing onto current HEAD. I’m sorry but it will take a day or so...

> +SET enable_mergejoin = off; -- planner choose MergeJoin even it has
> higher costs, so disable it for testing.
>
> This seems awfully strange. Why would the planner choose a plan if it
> had a higher cost?

I thought so but I couldn’t reproduce such situation now. I’ll investigate it more. If the issue has gone, I’ll remove that SET statement for straightforward tests.

>
> - * If the table or the server is configured to use remote estimates,
> - * identify which user to do remote access as during planning. This
> + * Identify which user to do remote access as during planning. This
> * should match what ExecCheckRTEPerms() does. If we fail due
> to lack of
> * permissions, the query would have failed at runtime anyway.
> */
> - if (fpinfo->use_remote_estimate)
> - {
> - RangeTblEntry *rte = planner_rt_fetch(baserel->relid, root);
> - Oid userid = rte->checkAsUser ?
> rte->checkAsUser : GetUserId();
> -
> - fpinfo->user = GetUserMapping(userid, fpinfo->server->serverid);
> - }
> - else
> - fpinfo->user = NULL;
> + rte = planner_rt_fetch(baserel->relid, root);
> + fpinfo->userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
>
> So, wait a minute, remote estimates aren't optional any more?

No, it seems to be removed accidentally. I’ll check the reason again though, but I’ll revert the change unless I find any problem.

--
Shigeru HANADA
shigeru(dot)hanada(at)gmail(dot)com