Hello, On Thu, Sep 10, 2026 at 7:05 AM Jinqing Kuang <[email protected]> wrote:
> > > On Sep 6, 2026, at 10:39, Sagar Shedge <[email protected]> wrote: > > > > Hi Hackers, > > > > add_foreign_final_paths() currently disables pushing down FETCH FIRST > > .. WITH TIES entirely, because doing so requires knowing whether the > > remote server is v13+ (which added support for the clause), and > > checking that would mean opening a connection during planning (see > > the discussion at > https://postgr.es/m/[email protected] > > which led to the current behavior). > > > > Attached patch fills in that one remaining gap. postgres_fdw already > > keeps a connection cache alive for the session's lifetime; if a > > connection to the relevant foreign server already exists in that cache > > (from an earlier query in the same session), its version is known for > > free, with no additional network access. GetCachedConnectionVersion() > > lookup into that cache and retun cached version. This information used in > > add_foreign_final_paths() to allow the pushdown only when a cached > > connection reports version 13 or later. The relation's server/user > > mapping are read from RelOptInfo's own serverid/userid fields, which > > are InvalidOid whenever the relation spans more than one foreign server > > (a cross-server join, or a sharded partitioned table). So the pushdown > > correctly stays disabled in those cases. > > > > appendLimitClause() is updated to emit the SQL-standard FETCH FIRST > > clause (with OFFSET ahead of it, per the grammar) instead of plain > > LIMIT/OFFSET when WITH TIES is in use. The value in that position is > > parsed as c_expr rather than a_expr, which does not accept the > > "::type" cast decoration deparseExpr() normally emits for constants; > > the patch parenthesizes it, which c_expr explicitly allows. > > > > Regarding the collation/tie-semantics concern raised in the original > > thread: by the time add_foreign_final_paths() runs, ORDER BY has > > already been determined safe to push down by an earlier check. Ties are > > just rows that compare equal under that same, already-vetted comparison. > > So no new risk is introduced by additionallyfetching the tied rows. > > > > Tested against a loopback foreign server, including: 1/ cold-cache > > sessions correctly falling back to local evaluation; 2/ warm-cache > > sessions pushing the FETCH clause down with results matching the > > non-FDW reference, both with and without OFFSET 3/ cross-server > > joins/unions correctly never attempting the pushdown. New regression > > tests added to postgres_fdw.sql/expected covering all of the above. > > make check passes. > > > > Regards, > > Sagar Shedge > > Multigres Engineer, Supabase > > > > <0001-postgres_fdw-fetch-first-with-ties.patch> > > Hi Sagar, > > I found two regressions in the patch. > > With use_remote_estimate=true, this fails during planning: > > SELECT a, count(*) FROM ft > WHERE b = 1 GROUP BY a, b > ORDER BY b FETCH FIRST 2 ROWS WITH TIES; > > The planner removes b from the sort keys because WHERE fixes its value. > The remote query then has WITH TIES without ORDER BY: > > ERROR: WITH TIES cannot be specified without ORDER BY clause > > ORDER BY (1+1) has the same problem on grouped queries. I’ve kept > WITH TIES local when pathkeys is empty. > > Ordinary EXPLAIN also fails with local estimates when the server has > neither a user-specific nor a PUBLIC mapping: > > CREATE SERVER no_mapping FOREIGN DATA WRAPPER postgres_fdw; > CREATE FOREIGN TABLE ft_no_mapping (a int) SERVER no_mapping; > EXPLAIN (VERBOSE, COST OFF) > SELECT a FROM ft_no_mapping ORDER BY a > FETCH FIRST 2 ROWS WITH TIES; > > GetUserMapping() errors before the cache lookup can fall back. I used > GetUserMappingExtended(..., DEBUG1) so a missing mapping keeps the limit > local. Existing mapping checks for remote estimates and execution still > apply. > > I’ve attached v2 with fixes for both cases on top of your original patch, > along with regression tests. > I gave the patch a quick review. It applies cleanly, builds, and make check in contrib/postgres_fdw passes, including the new tests. The logic looks correct to me, and I couldn't find a case where the pushdown produces different results than the local fallback. One thing worth discussing explicitly rather than leaving implicit is that the pushdown decision in add_foreign_final_paths() depends entirely on whatever connection happens to already be cached for that user mapping at plan time: if (user == NULL || GetCachedConnectionVersion(user) < 130000) return; That means the exact same query, planned twice in the same backend, can end up with two different plans purely because of unrelated activity in between: - First time a given foreign server is touched in a session (no cached connection yet) => WITH TIES stays local, *no pushdown*, the full result set for the ORDER BY gets fetched. - Any later query against that server in the same backend, once anything has opened a connection to it => *pushed down*. So EXPLAIN on the same statement can show a Foreign Scan with FETCH FIRST ... WITH TIES folded into the remote SQL on one run, and a local LIMIT node on another, with nothing about the query itself having changed. Someone diagnosing a slow query by comparing EXPLAIN output across sessions could easily mistake this for a bug. To be clear, I don't think this makes the patch wrong, but since it introduces a new source of connection-history-dependent plan shape in postgres_fdw, I think it's worth either: - a note in the code comment above the check (right now the comment explains why we use the cache, but not that this makes the pushdown decision session-history-dependent), and/or - a line in the commit message/release notes calling it out explicitly, so it doesn't surprise someone debugging plan differences later. Curious whether this tradeoff was already considered and just not written down, or whether there's a reason it's not worth documenting. Thanks > > Regards, > Jinqing > > -- *Jeevan Chalke* *Senior Principal Engineer, Engineering Manager* *Product Development* enterprisedb.com <https://www.enterprisedb.com>
