Re: [HACKERS] postgres_fdw bug in 9.6

On Sun, Dec 3, 2017 at 2:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Not sure where that leaves us for the patch at hand. It feels to me
> like it's a temporary band-aid for code that we want to get rid of
> in the not-too-long run. As such, it'd be okay if it were smaller,
> but it seems bigger and more invasive than I could wish for a band-aid.

Well, the bug report is a year old today, so we should try to do
something. The patch needs a rebase, but reading through it, I'm not
convinced that it's particularly risky. I mean, it's going to break
FDWs that are calling GetExistingLocalJoinPath(), but there are
probably very few third-party FDWs that do that. Any that do are
precisely the ones that need this fix, so I think there's a good
chance they'll forgive of us for requiring them to make a code change.
I think we can contain the risk of anything else getting broken to an
acceptable level because there's not really very much other code
changing. The changes to joinpath.c need to be carefully audited to
make sure that they are not changing the semantics, but that seems
like it shouldn't take more than an hour of code-reading to check.
The new fields in JoinPathExtraData can be moved to the end of the
struct to minimize ABI breakage. I don't see what else there is that
could break apart from the EPQ rechecks themselves, and if that
weren't broken already, this patch wouldn't exist.

Now, if you're still super-concerned about this breaking something, we
could commit it only to master, where it will have 9 months to bake
before it gets released. I think that's overly conservative, but I
think it's still better than waiting for the rewrite you'd like to see
happen. We don't know when or if anyone is going to undertake that,
and if we wait, we may easing release a v11 that's got the same defect
as v9.6 and now v10. And I don't see that we lose much by committing
this now even if that rewrite does happen in time for v11. Ripping
out CreateLocalJoinPath() won't be any harder than ripping out
GetExistingLocalJoinPath().

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company