Обсуждение: 9.5 release notes may need ON CONFLICT DO NOTHING compatibility notice for FDW authors
9.5 release notes may need ON CONFLICT DO NOTHING compatibility notice for FDW authors
От
Peter Geoghegan
Дата:
postgres_fdw supports ON CONFLICT DO NOTHING, provided no inference specification is provided. Foreign tables do not have associated unique indexes (or exclusion constraints) as far as the optimizer is concerned, and so Postgres does not accept an inference specification for foreign tables -- the optimizer will simply complain that a unique index that satisfies the user's inference specification is unavailable. There is no support for ON CONFLICT DO UPDATE with postgres_fdw, but that's really only because an inference specification (or explicitly named constraint) is always required for DO UPDATE. The deparsing support actually added will have deparsing add "ON CONFLICT DO NOTHING" for the SQL generated for execution on foreign servers if the original statement had that exact, unadorned ON CONFLICT clause. As things stand, every other possible ON CONFLICT clause will throw an error in some way before the FDW is consulted at all, so FDW authors need not concern themselves with those other cases (unless perhaps we allow ON CONFLICT DO UPDATE to not require an inference specification in a last minute behavioral tweak, as suggested by Simon Riggs, making ON CONFLICT DO UPDATE support by foreign data wrappers a possibility that must be considered). postgres_fdw handles the one simple ON CONFLICT DO NOTHING case (the only case that can actually reach it), but no other FDW we ship pay any attention. Do we need to make existing contrib FDWs, like file_fdw, explicitly reject unadorned ON CONFLICT DO NOTHING clauses as wrong-headed? Is it okay to just let them not pay attention at all on the theory that it's the same as performing INSERT ... ON CONFLICT DO NOTHING on a table that has no unique indexes or exclusion constraints? In any case, third party foreign data wrappers that target other database system will totally ignore ON CONFLICT DO NOTHING when built against the master branch (unless they consider these questions). They should perhaps make a point of rejecting DO NOTHING outright where it makes sense that support could exist, but it just doesn't. Or they could just add support (I imagine that this would be very easy for mysql_fdw, for example -- MySQL has INSERT IGNORE). I feel a compatibility item in the release notes is in order so the question is considered, but there seems to be no place to do that on the Wiki, and the original commit message does not have a note like this. -- Peter Geoghegan
Re: 9.5 release notes may need ON CONFLICT DO NOTHING compatibility notice for FDW authors
От
Peter Geoghegan
Дата:
On Sun, May 24, 2015 at 4:22 PM, Peter Geoghegan <pg@heroku.com> wrote: > As things stand, every other possible ON CONFLICT clause will throw an > error in some way before the FDW is consulted at all, so FDW authors > need not concern themselves with those other cases (unless perhaps we > allow ON CONFLICT DO UPDATE to not require an inference specification > in a last minute behavioral tweak, as suggested by Simon Riggs, making > ON CONFLICT DO UPDATE support by foreign data wrappers a possibility > that must be considered). AddForeignUpdateTargets() actually won't be called with ON CONFLICT DO UPDATE, and so it isn't exactly true that the only obstacle to making FDWs support ON CONFLICT DO UPDATE is around inference of arbiter unique indexes on the foreign side. It's *almost* true, though. -- Peter Geoghegan
Re: 9.5 release notes may need ON CONFLICT DO NOTHING compatibility notice for FDW authors
От
Peter Geoghegan
Дата:
On Sun, May 24, 2015 at 5:16 PM, Peter Geoghegan <pg@heroku.com> wrote: > AddForeignUpdateTargets() actually won't be called with ON CONFLICT DO > UPDATE, and so it isn't exactly true that the only obstacle to making > FDWs support ON CONFLICT DO UPDATE is around inference of arbiter > unique indexes on the foreign side. It's *almost* true, though. Attached patch clears this up within the fdw-handler documentation. I think it's worth separately noting from the existing commentary on limitations with FDWs and ON CONFLICT. -- Peter Geoghegan
Вложения
On 25 May 2015 at 00:22, Peter Geoghegan <pg@heroku.com> wrote:
--
There is no support for ON CONFLICT DO UPDATE with postgres_fdw, but
that's really only because an inference specification (or explicitly
named constraint) is always required for DO UPDATE. The deparsing
support actually added will have deparsing add "ON CONFLICT DO
NOTHING" for the SQL generated for execution on foreign servers if the
original statement had that exact, unadorned ON CONFLICT clause. As
things stand, every other possible ON CONFLICT clause will throw an
error in some way before the FDW is consulted at all, so FDW authors
need not concern themselves with those other cases (unless perhaps we
allow ON CONFLICT DO UPDATE to not require an inference specification
in a last minute behavioral tweak, as suggested by Simon Riggs, making
ON CONFLICT DO UPDATE support by foreign data wrappers a possibility
that must be considered).
My earlier summary was that the support for multiple constraints has been poorly thought through. This is an example of the breakage I have been complaining about when we are forced to specify the constraint (conflict-target).
This is not just related to FDWs and should not be fixed solely for FDWs. This was already an open item for me in 9.5, now even more so.
My comments do not come at the last minute, what Peter means is that we should make a change now in response to the concerns I have previously raised.
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Peter Geoghegan wrote: > In any case, third party foreign data wrappers that target other > database system will totally ignore ON CONFLICT DO NOTHING when built > against the master branch (unless they consider these questions). They > should perhaps make a point of rejecting DO NOTHING outright where it > makes sense that support could exist, but it just doesn't. Or they > could just add support (I imagine that this would be very easy for > mysql_fdw, for example -- MySQL has INSERT IGNORE). I feel a > compatibility item in the release notes is in order so the question is > considered, but there seems to be no place to do that on the Wiki, and > the original commit message does not have a note like this. +1 I wouldn't have become aware of that if I hadn't read your message. Yours, Laurenz Albe
On 2015/05/25 9:16, Peter Geoghegan wrote: > AddForeignUpdateTargets() actually won't be called with ON CONFLICT DO > UPDATE, and so it isn't exactly true that the only obstacle to making > FDWs support ON CONFLICT DO UPDATE is around inference of arbiter > unique indexes on the foreign side. It's *almost* true, though. I think that those are interesting problems. Wouldn't we need some additional hacks for the core or FDW to perform an operation that is equivalent to dynamically switching the ExecInsert/ExecForeignInsert processing to the ExecUpdate/ExecForeignUpdate processing in case of a conflict? Best regards, Etsuro Fujita
Re: Re: 9.5 release notes may need ON CONFLICT DO NOTHING compatibility notice for FDW authors
От
Peter Geoghegan
Дата:
On Thu, May 28, 2015 at 1:20 AM, Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> wrote: > I think that those are interesting problems. Wouldn't we need some > additional hacks for the core or FDW to perform an operation that is > equivalent to dynamically switching the ExecInsert/ExecForeignInsert > processing to the ExecUpdate/ExecForeignUpdate processing in case of a > conflict? I did not imagine so. Rather, I thought that it was a matter of simply introducing a way that foreign tables can have foreign constraints recognizable by the local Postgres optimizer. The decision to insert or update must belong to the foreign server, since the feature could be useful for systems like MySQL, and not just Postgres. I may be mistaken. -- Peter Geoghegan
Re: 9.5 release notes may need ON CONFLICT DO NOTHING compatibility notice for FDW authors
От
Peter Geoghegan
Дата:
On Mon, May 25, 2015 at 1:28 AM, Simon Riggs <simon@2ndquadrant.com> wrote: > My earlier summary was that the support for multiple constraints has been > poorly thought through. This is an example of the breakage I have been > complaining about when we are forced to specify the constraint > (conflict-target). > > This is not just related to FDWs and should not be fixed solely for FDWs. > This was already an open item for me in 9.5, now even more so. I agree that the decision to change the current behavior has nothing to do with FDWs. There is no reason to treat foreign tables differently to local ones in this regard, which implies that ON CONFLICT DO UPDATE cannot work with postgres_fdw unless and until someone invents foreign constraints on foreign tables (I think), or unless we change our mind generally (for other reasons). So, certainly, the rationale for mandating (or not mandating) an inference specification with ON CONFLICT DO UPDATE ought to come from balancing concerns about safety, compatibility, flexibility, and so on. I did not mean to imply that your comments were unreasonable/too late. However, I don't see a lot of demand for changing the behavior. There is at least some demand for accepting as arbiters multiple unique constraints (that are not more or less equivalent), from Andres for example, but that's a different question. It's also something that could reasonably be added later. -- Peter Geoghegan