On 18 August 2017 at 15:40, Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
> Ildar Musin wrote:
>
>> While we've been developing pg_pathman extension one of the most frequent
>> questions we got from our users was about global index support. We cannot
>> provide it within an extension. And I couldn't find any recent discussion
>> about someone implementing it. So I'm thinking about giving it a shot and
>> start working on a patch for postgres.
>>
>> One possible solution is to create an extended version of item pointer which
>> would store relation oid along with block number and position:
>
> I've been playing with the index code in order to allow indirect tuples,
> which are stored in a format different from IndexTupleData.
>
> I've been adding an "InMemoryIndexTuple" (maybe there's a better name)
> which internally has pointers to both IndexTupleData and
> IndirectIndexTupleData, which makes it easier to pass around the index
> tuple in either format.
> It's very easy to add an OID to that struct,
> which then allows to include the OID in either an indirect index tuple
> or a regular one.
If there is a unique index then there is no need for that. Additional
data to the index makes it even bigger and even less useful, so we
need to count that as a further disadvantage of global indexes.
I have a very clear statement from a customer recently that "We will
never use global indexes", based upon their absolute uselessness in
Oracle.
> Then, wherever we're using IndexTupleData in the index AM code, we would
> replace it with InMemoryIndexTuple. This should satisfy both your use
> case and mine.
Global indexes are a subset of indirect indexes use case but luckily
not the only use.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services