Re: [PROPOSAL] extend the object names to the qualified names inpg_stat_statements

On 11/29/2018 10:59 AM, Stephen Frost wrote:
> Greetings,
>
> * Alvaro Herrera (alvherre@2ndquadrant.com) wrote:
>> On 2018-Nov-28, Tom Lane wrote:
>>> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
>>>> On 2018-Nov-28, Tom Lane wrote:
>>>>> This would also entail rather significant overhead to find out schema
>>>>> names and interpolate them into the text.
>>>> True.  I was thinking that the qualified-names version of the query
>>>> would be obtained via ruleutils or some similar mechanism to deparse
>>>> from the parsed query tree (not from the original query text), where
>>>> only pg_catalog is considered visible.  This would be enabled using a
>>>> GUC that defaults to off.
>>> Color me skeptical --- ruleutils has never especially been designed
>>> to be fast, and I can't see that the overhead of this is going to be
>>> acceptable to anybody who needs pg_stat_statements in production.
>>> (Some admittedly rough experiments suggest that we might be
>>> talking about an order-of-magnitude slowdown for simple queries.)
>> Good point.
>>
>> Maybe we can save the OID array of schemas that are in search_path when
>> the query is first entered into the statement pool, and produce the
>> query_qn column only at the time the entry is interpreted (that is, when
>> pg_stat_statements is query).  ... oh, but that requires saving the plan
>> tree too, which doesn't sound very convenient.
>>
>> Maybe just storing the search_path schemas (as Tomas already suggested)
>> is sufficient for Sergei's use case?  Do away with query_qn as such, and
>> just have the user interpret the names according to the stored
>> search_path.
> Seems like what you'd really want is to store all the environment, not
> just the search_path (consider the $user case...).  Maybe saving just
> the OIDs of the search_path and then using them later would also work
> but it seems like we're just building up to tracking everything and
> doing it piecemeal with an extra column added in this release, another
> in the next release, etc..
>
> Thanks!
>
> Stephen
It's a valid concern. Instead of the adding just search_path column we 
can add a column session_info jsonb.
Its content can be defined by the new configuration parameter

pg_stat_statements.session_info ('current_schemas, current_user, 
session_user') // a subset of the data from the system information functions

and it will have data like
{
     "current_schemas" : ["pg_catalog", "s1", "s2", "public"],
     "current_user" : "user1",
     "session_user" : "user1"
}

It will allow the DBA/developer to reproduce a performance issue, and 
will allow the deeper level of granularity for the reporting tools.
It's more complex than I have anticipated but doesn't break backward 
compatibility and extensible.

Thank you,

Sergei