On 13.12.2011 11:57, Albe Laurenz wrote:
Tom Lane wrote:
Shigeru Hanada<shigeru.han...@gmail.com>  writes:
(2011/12/12 22:59), Robert Haas wrote:
... I feel like we might need a system here that
allows for more explicit user control about what to push down vs.
rather than assuming we'll be able to figure it out behind the

Agreed.  How about to add a per-column boolean FDW option, say
"pushdown", to pgsql_fdw?  Users can tell pgsql_fdw that the column
be pushed down safely by setting this option to true.

[ itch... ] That doesn't seem like the right level of granularity.
ISTM the problem is with whether specific operators have the same
meaning at the far end as they do locally.  If you try to attach the
flag to columns, you have to promise that *every* operator on that
column means what it does locally, which is likely to not be the
case ever if you look hard enough.  Plus, having to set the flag on
each individual column of the same datatype seems pretty tedious.

I don't have a better idea to offer at the moment though.  Trying
to attach such a property to operators seems impossibly messy too.
If it weren't for the collations issue, I might think that labeling
datatypes as being compatible would be a workable approximation.

Maybe I'm missing something, but if pushdown worked as follows:

- Push down only system functions and operators on system types.
- Only push down what is guaranteed to work.

then the only things we would miss out on are encoding- or
collation-sensitive string operations.

Is that loss so big that it warrants a lot of effort?

The SQL/MED spec handles this with the concept of "routine mappings". There is syntax for defining which remote "routines", meaning functions, correspond local functions:

CREATE ROUTINE MAPPING <routine mapping name> FOR <specific routine designator>
SERVER <foreign server name> [ <generic options> ]

<generic options> is FDW-specific, I'd imagine the idea is to give the name of the corresponding function in the remote server. It doesn't say anything about collations, but you could have extra options to specify that a function can only be mapped under C collation, or whatever.

It seems tedious to specify that per-server, though, so we'll probably still want to have some smarts in the pgsql_fdw to handle the built-in functions and types that we know to be safe.

I've been talking about functions here, not operators, on the assumption that we can look up the function underlying the operator and make the decisions based on that.

  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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