Hi,
On 2019-07-27 18:29:23 +0000, Daniel Migowski wrote:
> I just implemented a small change that adds another column "mem_usage"
> to the system view "pg_prepared_statements". It returns the memory
> usage total of CachedPlanSource.context,
> CachedPlanSource.query_content and if available
> CachedPlanSource.gplan.context.
FWIW, it's generally easier to comment if you actually provide the
patch, even if it's just POC, as that gives a better handle on how much
additional complexity it introduces.
I think this could be a useful feature. I'm not so sure we want it tied
to just cached statements however - perhaps we ought to generalize it a
bit more.
Regarding the prepared statements specific considerations: I don't think
we ought to explicitly reference CachedPlanSource.query_content, and
CachedPlanSource.gplan.context.
In the case of actual prepared statements (rather than oneshot plans)
CachedPlanSource.query_context IIRC should live under
CachedPlanSource.context. I think there's no relevant cases where
gplan.context isn't a child of CachedPlanSource.context either, but not
quite sure.
Then we ought to just include child contexts in the memory computation
(cf. logic in MemoryContextStatsInternal(), although you obviously
wouldn't need all that). That way, if the cached statements has child
contexts, we're going to stay accurate.
> Also I wonder why the "prepare test as" is part of the statement
> column. I isn't even part of the real statement that is prepared as
> far as I would assume. Would prefer to just have the "select *..." in
> that column.
It's the statement that was executed. Note that you'll not see that in
the case of protocol level prepared statements. It will sometimes
include relevant information, e.g. about the types specified as part of
the prepare (as in PREPARE foo(int, float, ...) AS ...).
Greetings,
Andres Freund
