On Thu, Dec 26, 2024 at 06:58:11PM +0100, Tomas Vondra wrote:
> If 128MB is insufficient, why would 256MB be OK? A factor of 2x does not
> make a fundamental difference ...
> 
> Anyway, the 128MB value is rather arbitrary. I don't mind increasing the
> limit, or possibly removing it entirely (and accepting anything the
> system can handle).

+    DefineCustomIntVariable("stats_history.size",
+                            "Sets the amount of memory available for past 
events.",

How about some time-based retention?  Data size can be hard to think
about for the end user, while there would be a set of users that would
want to retain data for the past week, month, etc?  If both size and
time upper-bound are define, then entries that match one condition or
the other are removed.

+        checkpoint_log_hook(
+          CheckpointStats.ckpt_start_t,            /* start_time */
+          CheckpointStats.ckpt_end_t,                /* end_time */
+          (flags & CHECKPOINT_IS_SHUTDOWN),        /* is_shutdown */
+          (flags & CHECKPOINT_END_OF_RECOVERY),    /* is_end_of_recovery */
+          (flags & CHECKPOINT_IMMEDIATE),            /* is_immediate */
+          (flags & CHECKPOINT_FORCE),                /* is_force */
+          (flags & CHECKPOINT_WAIT),                /* is_wait */
+          (flags & CHECKPOINT_CAUSE_XLOG),        /* is_wal */
+          (flags & CHECKPOINT_CAUSE_TIME),        /* is_time */
+          (flags & CHECKPOINT_FLUSH_ALL),            /* is_flush_all */
+          CheckpointStats.ckpt_bufs_written,        /* buffers_written */
+          CheckpointStats.ckpt_slru_written,        /* slru_written */
+          CheckpointStats.ckpt_segs_added,        /* segs_added */
+          CheckpointStats.ckpt_segs_removed,        /* segs_removed */
+          CheckpointStats.ckpt_segs_recycled,        /* segs_recycled */

That's a lot of arguments.  CheckpointStatsData and the various
CHECKPOINT_* flags are exposed, why not just send these values to the
hook?

For v1-0001 as well, I'd suggest some grouping with existing
structures, or expose these structures so as they can be reused for
out-of-core code via the proposed hook.  More arguments lead to more
mistakes that could be easily avoided.
--
Michael

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