On Mon, Dec 12, 2016 at 5:45 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> One way that we could make things better is to rely on the knowledge
> that EPQ isn't asked to evaluate joins for more than one row per input
> relation, and therefore using merge or hash join technology is really
> overkill.  We could make a tree of simple nestloop joins, which aren't
> going to care about sort order, if we could identify the correct join
> clauses to apply.  At least some of that could be laid off on the FDW,
> which if it's gotten this far at all, ought to know what join clauses
> need to be enforced by the foreign join.  So I'm thinking a little bit
> in terms of "just collect the foreign scans for all the base rels
> included in the join and then build a cross-product nestloop join tree,
> applying all the join clauses at the top".  This would have the signal
> value that it's guaranteed to succeed and so can be left for later,
> rather than having to expensively redo it at each level of join planning.
> (Hm, that does sound a lot like "throw it away and start again", doesn't
> it.  But what we've got here is busted enough that I'm not sure there
> are good alternatives.  Maybe for 9.6 we could just rewrite
> GetExistingLocalJoinPath, and limp along doing a lot of redundant
> computation during planning.)

I think there was an earlier version of the patch that actually built
up NestLoop joins as it went, but at some point (possibly at my
suggestion) it got rewritten to try to fish out existing paths
instead.  Apparently, the other idea was better.

> BTW, what's "existing" about the result of GetExistingLocalJoinPath?

It's an existing path - i.e. already added to the RelOptInfo - to
perform the join locally.

> And for that matter, what's "outer" about the content of fdw_outerpath?

It's got the same meaning here that "outer path" does in general.
Right now, a Foreign Scan only has an outer subpath if it needs one
for EPQ rechecks, but in theory I suppose it could be used for
something else; every Plan has a lefttree, whether or not we choose to
populate it.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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