Re: Proposal: Global Index for PostgreSQL

Hi Dilip,

Happy to see you working on this. It’s clear a lot of thought has
gone into the design.

On Tue, Jul 1, 2025 at 6:27 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> 6) Need to perform a performance test, for SELECT/UPDATE/INSERT cases,
> we already know the VACUUM performance.

One point I want to check my understanding of is around the locking
implications of global index scans, especially in prepared statement
scenarios.

I’ve been working on improving how we handle partition locking during
execution of generic plans. Specifically, I committed a patch to defer
locking of partitions until after pruning during ExecutorStart(), so
we avoid taking locks on partitions that aren’t actually needed --
even when the plan contains scans on all partitions. That patch was
later reverted, as Tom pointed out that the plan invalidation logic
wasn't cleanly handled. But the goal remains: to avoid locking
unnecessary partitions, particularly in high-partition-count OLTP
setups that use PREPARE/EXECUTE.

The proposed global index design, IIUC, requires locking all leaf
partitions up front during planning, and I guess during
AcquireExecutorLocks() when using a cached plan, because the index
scan could return tuples from any partition. This seems to directly
undercut that effort: we'd be back to generic plans causing broad
locking regardless of actual runtime needs.

I understand that this is currently necessary, given that a global
index scan is a single node without per-partition awareness. But it
might be worth considering whether the scan could opportunistically
defer heap relation locking until it returns a tuple that actually
belongs to a particular partition -- similar to how inserts into
partitioned tables only lock the target partition at execution time.
Or did I miss that inserts also need to lock all partitions up front
when global indexes are present, due to cross-partition uniqueness
checks?

Let me know if I’ve misunderstood the design.

--
Thanks, Amit Langote