1. For a regular FDW scan, zero the xmin, xmax, and cid of the tuple before returning it from postgres_fdw, so that we don't expose the datum-tuple fields. I can't see any reason this isn't safe, but I might be missing something.
2. When a join is pushed down, deparse system columns using something like "CASE WHEN r1.* IS NOT NULL THEN 0 END", except for the table OID column, which gets deparsed with the table OID in place of 0. This delivers the correct behavior in the presence of outer joins.
Review appreciated.
It looks good to me. It might be good to explain why we don't add CASE .. END statement when qualify_col is false, i.e. "qualify_col being false indicates that there is only one relation involved thus no join."
BTW, I noticed that we are deparsing whole-row reference as ROW(list of columns from local definition of foreign table), which has the same problem with outer joins. It won't be NULL when the rest of the row from that relation is NULL in an outer join. It too needs to be encapsulated in CASE WHEN .. END expression. PFA patch with that fix included and also some testcases for system columns as well as whole-row references.
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Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company
