Re: [PATCH] Increase the maximum value track_activity_query_size

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.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services