This command provides an automated way to create foreign table definitions
that match remote tables, thereby reducing tedium and chances for error.
In this patch, we provide the necessary core-server infrastructure and
implement the feature fully in the postgres_fdw foreign-data wrapper.
Other wrappers will throw a "feature not supported" error until/unless
they are updated.
Ronan Dunklau and Michael Paquier, additional work by me
The previous coding supposed that it could consider just a single join
condition in any one parameterized path for the foreign table. But in
reality, the parameterized-path machinery forces all join clauses that are
"movable to" the foreign table to be evaluated at that node; including
clauses that we might not consider safe to send across. Such cases would
result in an Assert failure in an assert-enabled build, and otherwise in
sending an unsafe clause to the foreign server, which might result in
errors or silently-wrong answers. A lesser problem was that the
cost/rowcount estimates generated for the parameterized path failed to
account for any additional join quals that get assigned to the scan.
To fix, rewrite postgresGetForeignPaths so that it correctly collects all
the movable quals for any one outer relation when generating parameterized
paths; we'll now generate just one path per outer relation not one per join
qual. Also fix bogus assumptions in postgresGetForeignPlan and
estimate_path_cost_size that only safe-to-send join quals will be
presented.
Based on complaint from Etsuro Fujita that the path costs were being
miscalculated, though this is significantly different from his proposed
patch.
postgres_fdw tended to say "unknown error" if it tried to execute a command
on an already-dead connection, because some paths in libpq just return a
null PGresult for such cases. Out-of-memory might result in that, too.
To fix, pass the PGconn to pgfdw_report_error, and look at its
PQerrorMessage() string if we can't get anything out of the PGresult.
Also, fix the transaction-exit logic to reliably drop a dead connection.
It was attempting to do that already, but it assumed that only connection
cache entries with xact_depth > 0 needed to be examined. The folly in that
is that if we fail while issuing START TRANSACTION, we'll not have bumped
xact_depth. (At least for the case I was testing, this fix masks the
other problem; but it still seems like a good idea to have the PGconn
fallback logic.)
Per investigation of bug #9087 from Craig Lucas. Backpatch to 9.3 where
this code was introduced.
This should provide some marginal overall savings, since it surely takes
many more cycles for the remote server to deal with the NULL columns than
it takes for postgres_fdw not to emit them. But really the reason is to
keep the emitted queries from looking quite so silly ...
I wasn't going to ship this without having at least some example of how
to do that. This version isn't terribly bright; in particular it won't
consider any combinations of multiple join clauses. Given the cost of
executing a remote EXPLAIN, I'm not sure we want to be very aggressive
about doing that, anyway.
In support of this, refactor generate_implied_equalities_for_indexcol
so that it can be used to extract equivalence clauses that aren't
necessarily tied to an index.
Adopt the position that only locally-defined defaults matter. Any defaults
defined in the remote database do not affect insertions performed through
a foreign table (unless they are for columns not known to the foreign
table). While it'd arguably be more useful to permit remote defaults to be
used, making that work in a consistent fashion requires far more work than
seems possible for 9.3.
For datatypes whose output formatting depends on one or more GUC settings,
we have to worry about whether the other server will interpret the value
the same way it was meant. pg_dump has been aware of this hazard for a
long time, but postgres_fdw needs to deal with it too. To fix data
retrieval from the remote server, set the necessary remote GUC settings at
connection startup. (We were already assuming that settings made then
would persist throughout the remote session.) To fix data transmission to
the remote server, temporarily force the relevant GUCs to the right values
when we're about to convert any data values to text for transmission.
This is all pretty grotty, and not very cheap either. It's tempting to
think of defining one uber-GUC that would override any settings that might
render printed data values unportable. But of course, older remote servers
wouldn't know any such thing and would still need this logic.
While at it, revert commit f7951eef89, since
this provides a real fix. (The timestamptz given in the error message
returned from the "remote" server will now reliably be shown in UTC.)
This patch adds the core-system infrastructure needed to support updates
on foreign tables, and extends contrib/postgres_fdw to allow updates
against remote Postgres servers. There's still a great deal of room for
improvement in optimization of remote updates, but at least there's basic
functionality there now.
KaiGai Kohei, reviewed by Alexander Korotkov and Laurenz Albe, and rather
heavily revised by Tom Lane.
Include eval costs of local conditions in remote-estimate mode, and don't
assume the remote eval cost is zero in local-estimate mode. (The best
we can do with that at the moment is to assume a seqscan, which may well
be wildly pessimistic ... but zero won't do at all.)
To get a reasonable local estimate, we need to know the relpages count
for the remote rel, so improve the ANALYZE code to fetch that rather
than just setting the foreign table's relpages field to zero.
There's still a lot of room for improvement, but it basically works,
and we need this to be present before we can do anything much with the
writable-foreign-tables patch. So let's commit it and get on with testing.
Shigeru Hanada, reviewed by KaiGai Kohei and Tom Lane
Bruce Momjian
Alvaro Herrera
Tom Lane