Re: Better shared data structure management and resizable shared data structures

On Sun, Mar 22, 2026 at 5:44 AM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>
>
> I split this into more incremental patches. The first few are just
> refactorings that are probably useful on their own.

Here are some comments on the patches

0001
+
+# Enable pg_stat_statements to test restart of shared_preload_libraries.
+$node->append_conf(
+ 'postgresql.conf',
+ qq{shared_preload_libraries = 'pg_stat_statements'
+pg_stat_statements.max = 50000
+compute_query_id = 'regress'
+});
+

In order to make sure that the shared memory and LWLocks for
pg_stat_statements are initialized after crash restart, we need to at
least query pg_stat_statements after the restart or do something so
that it's shared memory is used. Also, we can check whether the shared
memory structures are created by querying pg_shmem_allocations after
the restart.

0002
void
InitShmemAllocator(PGShmemHeader *seghdr)
{

The new ShmemIndex initialization code is cleaner and more
straightforward. It avoids the recursive nature of ShmemInitHash.
However with this change it's hard to keep track of all the
initialization steps and their dependencies. Attached is a patch that
makes small adjustments to the code to make it more clear.

Use of variable hash_size is actually misleading since it's not the
size of the hash table but the expected/max number of entries in it.
Removing it makes code more readable.

0003, 0005, 0006 is straight forward, no comments. Usually these
patches make the code more readable. I will review it more when I see
the patches that use this refactoring.

0004 I traced back the placement of CreateLWLock in history. It feels
like the patch is moving it to its intended place since the function
was introduced. It needs a comment to explain why the function is not
called from CreateOrAttachShmemStructs where other in-memory
structures are allocated.

0007 without using new APIs is not necessarily a win. So I would
suggest committing it along with the refactoring patch.

If we are going to commit these patches separately, I would suggest
squashing all predicate.c patches into one commit.

I will continue from 0008 tomorrow.

--
Best Wishes,
Ashutosh Bapat