Re: [HACKERS] MERGE SQL Statement for PG11

On 24 March 2018 at 12:16, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Mar 22, 2018 at 7:13 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>> While I think this this particular HINT buglet is pretty harmless, I
>> continue to be concerned about the unintended consequences of having
>> multiple RTEs for MERGE's target table. Each RTE comes from a
>> different lookup path -- the first one goes through setTargetTable()'s
>> parserOpenTable() + addRangeTableEntryForRelation() calls. The second
>> one goes through transformFromClauseItem(), for the join, which
>> ultimately ends up calling transformTableEntry()/addRangeTableEntry().
>> Consider commit 5f173040, which fixed a privilege escalation security
>> bug around multiple name lookup. Could the approach taken by MERGE
>> here introduce a similar security issue?
>
> Yeah, that seems really bad. I don't think there is a huge problem
> with having multiple RTEs; for example, we very commonly end up with
> both rte->inh and !rte->inh RTEs for the same table, and that is OK.
> However, generating those RTEs by doing multiple name lookups for the
> same table is a big problem. Imagine, for example, that a user has a
> search_path of a, b and that there is a table b.foo. The user does a
> merge on foo. Between the first name lookup and the second, somebody
> creates a.foo. Now, potentially, half of the MERGE statement is going
> to be running against b.foo and the other half against a.foo. I don't
> know whether that will crash or bomb out with a strange error or just
> make some unexpected modification to one of those tables, but the
> behavior, even if not insecure, will certainly be wrong.

MERGE uses multiple RTEs in exactly the same way UPDATE does.

I don't see a reason for specific concern wrt to MERGE.

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