Enum in foreign table: error and correct way to handle.

On 21/05/15 04:23, Peter Swartz wrote:

I'm creating a foreign table (foo_table) in database_a. foo_table lives in
database_b.foo_table has an enum (bar_type) as one of its columns. Because
this enum is in database_b, the creation of the foreign table fails in
database_a. database_a doesn't understand the column type. Running the
following in database_a

CREATE FOREIGN TABLE foo_table (id integer NOT NULL, bar bar_type) SERVER
database_b

One gets the error:

ERROR: type "bar_type" does not exist

I could just create a copy of bar_type in database_a, but this feels
duplicative and possibly a future cause of inconsistency / trouble. Would
anyone have thoughts on best practices for handling?

A foreign table is basically an ad-hoc remote data source for the local database,
so the onus is on the local database to maintain its definition of the
remote table, whether it's in another (or even the same) PostgreSQL server
or a completely different data source, especially as the local definition can
be different from the remote one.

This does mean that there's no simple way of ensuring any remote dependencies are
present on the local server. PostgreSQL 9.5 will provide the IMPORT FOREIGN SCHEMA
command, however this is limited to table/view definitions.

Regards

Ian Barwick

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Thank you for the note Ian. I definitely see your point about the onus
being on the local database to maintain the definition of the remote
table. Do you or anyone have this list have any experience with the
resulting behavior if the definition of the enum were to become out of sync
between the local database and the foreign database? In other words,
suppose the foreign database adds a value to the enum, and the foreign
table now has rows with this new value, while the local definition of the
enum remains unchanged. Obviously, the appropriate action on my part is to
maintain consistency of enum definition between the foreign and local
database, but I'm curious about what behavior would result if there was an
error in this manual updating process.

I may dig into this a bit further myself in a few test databases, to see
what happens. Will post a response if I do.

With regards,
Peter

On Thu, May 21, 2015 at 8:06 PM, Ian Barwick <ian@2ndquadrant.com> wrote:

Peter Swartz <peter.goodings.swartz@gmail.com> writes:

suppose the foreign database adds a value to the enum, and the foreign
table now has rows with this new value, while the local definition of the
enum remains unchanged. Obviously, the appropriate action on my part is to
maintain consistency of enum definition between the foreign and local
database, but I'm curious about what behavior would result if there was an
error in this manual updating process.

What I'd expect to happen is that you'd get errors when retrieving rows
that had the values not known on the local side.

One potential way around this is to declare the foreign table's columns
as "text" rather than enums; you would lose some error checking on the
local side, but the remote server would enforce validity whenever you
stored something. (But I'm not sure whether this hack behaves desirably
for WHERE conditions on the enum column; you'd need to test.)

regards, tom lane

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Thank you for the message Tom; sounds great. I'll try that out, will check
on the planner's resultant behavior and email back.

Peter

On Sat, May 23, 2015 at 12:35 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote: