Re: Custom/Foreign-Join-APIs (Re: [v9.5] Custom Plan API)

On Tue, Jan 6, 2015 at 9:17 AM, Kouhei Kaigai <kaigai@ak.jp.nec.com> wrote:
> The attached patch is newer revision of custom-/foreign-join
> interface.

It seems that the basic purpose of this patch is to allow a foreign
scan or custom scan to have scanrelid == 0, reflecting the case where
we are scanning a joinrel rather than a baserel.  The major problem
that seems to create is that we can't set the target list from the
relation descriptor, because there isn't one.  To work around that,
you've added fdw_ps_list and custom_ps_tlist, which the FDW or
custom-plan provider must set.  I don't know off-hand whether that's a
good interface or not.  How does the FDW know what to stick in there?
There's a comment that seems to be trying to explain this:

+ * An optional fdw_ps_tlist is used to map a reference to an attribute of
+ * underlying relation(s) on a pair of INDEX_VAR and alternative varattno.
+ * It looks like a scan on pseudo relation that is usually result of
+ * relations join on remote data source, and FDW driver is responsible to
+ * set expected target list for this. If FDW returns records as foreign-
+ * table definition, just put NIL here.

...but I can't understand what that's telling me.

You've added an "Oid fdw_handler" field to the ForeignScan and
RelOptInfo structures.  I think this is the OID of the pg_proc entry
for the handler function; and I think we need it because, if scanrelid
== 0 then we don't have a relation that we can trace to a foreign
table, to a server, to an FDW, and then to a handler.  So we need to
get that information some other way.  When building joinrels, the
fdw_handler OID, and the associated routine, are propagated from any
two relations that share the same fdw_handler OID to the resulting
joinrel.  I guess that's reasonable, although it feels a little weird
that we're copying around both the OID and the structure-pointer.

For non-obvious reasons, you've made create_plan_recurse() non-static.

-- 
Robert Haas
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