Re: [PATCH] Increase the maximum value track_activity_query_size

On Sun, Dec 22, 2019 at 1:03 AM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
>
> On Sat, Dec 21, 2019 at 04:25:05PM -0500, Tom Lane wrote:
> >Nikolay Samokhvalov <samokhvalov@gmail.com> writes:
> >> Here is what ORMs do:
> >> select length('SELECT "column_name_1001", "column_name_1002",
> >> "column_name_1003", "column_name_1004", "column_name_1005",
> >> "column_name_1006", "column_name_1007", "column_name_1008",
> >> "column_name_1009", "column_name_1010", "column_name_1011",
> >> "column_name_1012", "column_name_1013", "column_name_1014",
> >> "column_name_1015", "column_name_1016", "column_name_1017",
> >> "column_name_1018", "column_name_1019", "column_name_1020",
> >> "column_name_1021", "column_name_1022", "column_name_1023",
> >> "column_name_1024", "column_name_1025", "column_name_1026",
> >> "column_name_1027", "column_name_1028", "column_name_1029",
> >> "column_name_1030", "column_name_1031", "column_name_1032",
> >> "column_name_1033", "column_name_1034", "column_name_1035",
> >> "column_name_1036", "column_name_1037", "column_name_1038",
> >> "column_name_1039", "column_name_1040", "column_name_1041",
> >> "column_name_1042", "column_name_1043", "column_name_1044",
> >> "column_name_1045", "column_name_1046", "column_name_1047",
> >> "column_name_1048", "column_name_1049", "column_name_1050" FROM
> >> "some_table";');
> >>  length
> >> --------
> >>    1024
> >> (1 row)
> >
> >> That's it – with default settings, you won't see WHERE clause or
> >> anything else.
> >
> >If that's true, it doesn't offer much of a case for upping the limit
> >on track_activity_query_size.  The longest such a query could reasonably
> >get is somewhere near NAMEDATALEN times MaxHeapAttributeNumber, which
> >as it happens is exactly the existing limit on track_activity_query_size.
> >
> >> As a result, many queries exceed track_activity_query_size
> >
> >How?  And if they are, why do you care?  Such queries sure seem
> >pretty content-free.
> >
>
> I believe the example was just a very simplistic example. ORMs can of
> course generate queries with joins, which can easily exceed the limit
> you mentioned.
>
> >> What is the overhead here except the memory consumption?
> >
> >The time to copy those strings out of shared storage, any time
> >you query pg_stat_activity.
> >
>
> IMO that seems like a reasonable price to pay, if you want to see
> complete queries and bump the track_activity_query_size value up.

Couldn't be pg_stat_statements (or any similar extension) queryid
exposure in pg_stat_activity [1] also an alternative?  You wouldn't
have the parameters but maybe the normalized query would be enough for
most analysis.  Now, maybe pg_stat_statements jumble overhead for such
large statements would be even more problematic.

[1] https://commitfest.postgresql.org/26/2069/