Hi hackers,
Adding Melanie and Tomas to this thread. I noticed you both recently committed similar changes to handle shared memory instrumentation separation and retrieval for IndexScan/IndexOnlyScan (
dd78e69cfc) and SeqScan (
3b1117d6e2).
My patch implements the exact same pattern for CustomScan states. It adds an optional RetrieveInstrumentationCustomScan hook so that extensions can aggregate their worker metrics before the DSM gets unlinked, bringing custom scans to parity with the recent core scan instrumentation improvements.
Could you please help review?
The CommitFest entry is also updated here:
https://commitfest.postgresql.org/patch/6524/Thanks, Siddharth
Hi hackers,
Bumping this thread as this small patch is passing all CF checks but needs a reviewer.
The Problem: Right now, if you write a parallel CustomScan extension, it is impossible to get performance metrics from parallel workers. The data is destroyed during DSM unlinking before the leader can grab it.
The Fix: This patch adds an optional RetrieveInstrumentation hook to CustomExecMethods. It has zero overhead for existing extensions (it's a simple NULL check) but unlocks metric aggregation for parallel custom scans.
Please let me know your thoughts.
Thanks,
Siddharth
Hi everyone,
I’m just checking in to see if anyone has had a chance to look at this or if there’s any further information I should provide to help with the review. I have also added the patch to PG20-1 CF queue, the link is
https://commitfest.postgresql.org/patch/6524/.
Thanks,
Siddharth
Dear PostgreSQL Hackers,
This email proposes a patch to enhance the CustomScan provider interface. The patch file, 0001-Add-RetrieveInstrumentationCustomScan-hook-for-Custo.patch, is attached.
Problem:
CustomScan providers currently lack a standard method to aggregate instrumentation data from parallel workers back to the leader process before the Dynamic Shared Memory (DSM) segment is unlinked. This makes it difficult to gather comprehensive performance metrics from parallelized custom scans.
Solution:
This patch introduces a new optional hook, RetrieveInstrumentationCustomScan, to the CustomExecMethods struct. This hook allows custom scan providers to implement logic to collect and consolidate instrumentation from shared memory or worker states during the parallel query cleanup phase. This hook is invoked via the new ExecCustomScanRetrieveInstrumentation function, called from ExecParallelRetrieveInstrumentation for T_CustomScanState nodes. Since the hook is optional (checked for NULL before calling), it maintains full backward compatibility.
Testing & Compatibility:
- The patch compiles and passes all core regression tests (
make check-world) on my x86_64 instance. - The changes are not platform-specific.
- Regression Tests: This patch provides a new capability for custom scan providers. Since the hook's functionality is only realized when implemented by an extension, specific tests would naturally reside within that extension rather than in the core regression suite.
This patch does not directly address a specific item on the official TODO list but enhances the extensibility framework.I believe this patch is complete and ready for review. I look forward to any feedback and am happy to make revisions. I will also add this patch to the next CommitFest.
Thank you,
Siddharth Kothari
