Re: Case expression pushdown - Mailing list pgsql-hackers

Gilles Darold <gilles@migops.com> writes:
> I'm attaching the v5 patch again as it doesn't appears in the Latest 
> attachment list in the commitfest.

So this has a few issues:

1. In foreign_expr_walker, you're failing to recurse to either the
"arg" or "defresult" subtrees of a CaseExpr, so that it would fail
to notice unshippable constructs within those.

2. You're also failing to guard against the hazard that a WHEN
expression within a CASE-with-arg has been expanded into something
that doesn't look like "CaseTestExpr = something".  As written,
this patch would likely dump core in that situation, and if it didn't
it would send nonsense to the remote server.  Take a look at the
check for that situation in ruleutils.c (starting at line 8764
as of HEAD) and adapt it to this.  Probably what you want is to
just deem the CASE un-pushable if it's been modified away from that
structure.  This is enough of a corner case that optimizing it
isn't worth a great deal of trouble ... but crashing is not ok.

3. A potentially uncomfortable issue for the CASE-with-arg syntax
is that the specific equality operator being used appears nowhere
in the decompiled expression, thus raising the question of whether
the remote server will interpret it the same way we did.  Given
that we restrict the values-to-be-compared to be of shippable
types, maybe this is safe in practice, but I have a bad feeling
about it.  I wonder if we'd be better off just refusing to ship
CASE-with-arg at all, which would a-fortiori avoid point 2.

4. I'm not sure that I believe any part of the collation handling.
There is the question of what collations will be used for the
individual WHEN comparisons, which can probably be left for
the recursive checks of the CaseWhen.expr subtrees to handle;
and then there is the separate issue of whether the CASE's result
collation (which arises from the CaseWhen.result exprs plus the
CaseExpr.defresult expr) can be deemed to be safely derived from
remote Vars.  I haven't totally thought through how that should
work, but I'm pretty certain that handling the CaseWhen's within
separate recursive invocations of foreign_expr_walker cannot
possibly get it right.  However, you'll likely have to flatten
those anyway (i.e., handle them within the loop in the CaseExpr
case) while fixing point 2.

5. This is a cosmetic point, but: the locations of the various
additions in deparse.c seem to have been chosen with the aid
of a dartboard.  We do have a convention for this sort of thing,
which is to lay out code concerned with different node types
in the same order that the node types are declared in *nodes.h.
I'm not sufficiently anal to want to fix the existing violations
of that rule that I see in deparse.c; but the fact that somebody
got this wrong before isn't license to make things worse.

            regards, tom lane