Re: [HACKERS] BUG: pg_stat_statements query normalization issueswith combined queries

Hello,

At Fri, 30 Dec 2016 15:10:42 +0100 (CET), Fabien COELHO <coelho@cri.ensmp.fr> wrote in
<alpine.DEB.2.20.1612301453280.32017@lancre>
> 
> Hello,
> 
> >> Yeah, that's what I was thinking of.  There aren't very many places
> >> that
> >> would need to know about that, I believe; [...]
> >
> > For fixing the information in pg_stat_statement, the location data
> > must be transported from the parsed node to the query to the planned
> > node, because the later two nodes types are passed to different hooks.
> >
> > Now the detail is that utility statements, which seems to be nearly
> > all of them but select/update/delete/insert, do not have plans: The
> > statement itself is its own plan... so there is no place to store the
> > location & length.
> 
> Here is an updated version:
> 
> Changes wrt v2:
> 
>  - I have added the missing stuff under /nodes/, this is stupid code that
>    should be automatically generated:-(
>
>  - I have added comments in "gram.y" about how the length is computed.
>    I have also slightly simplified the rule code there.
> 
>  - I have rename "location" in create table space to "location_dir"
>    to avoid confusion.
> 
>  - I have renamed the fields "location" and "length" instead of q*.
> 
>  - I have moved the location & lenth copies in standard_planner.
> 
>  - I have fixed the function declaration typo.
> 
>  - I have simplified pgss_store code to avoid a copy, and move the
>    length truncation in qtext_store.
> 
>  - I have improved again the pg_stat_statement regression tests with
>    combined utility statement tests, which implied some fixes to
>    extract the right substring for utility queries.
> 
> However, not changed:
> 
>  - I cannot use the intermediate node trick suggested by Tom because
>    it does not work for utility statements which do not have plans, so
>    the code still adds location & length, sorry.

Wrapping the output of pg_parse_query still works for non-utility
commands. But pg_stat_statement retrieves the location
information via ProcessUtility, which has plantree. Wrapping
plantree with the same struct also works but it imacts too widely
and extremely error-prone.

One available solution is, as Fabien did, bring location and
length data along with transformation.  And anther is give a
chopped query to query tree.

The attached patch does the second way (but quite ugly and
incomplete, it's PoC). This seems working as expected to a
certain degree but somewhat bogus..  A significant drawback of
this is that this makes a copy of each query in multistatement.

>  - I still use the 'last_semicolon' lexer variable. The alternative is to
>    change rules so as not to skip empty statements, then write a loop to
>    compute the length based on successor location, and remove the empty
>    statements. It can be done, I do not think it is better, it is only
>    different and more verbose. I'll do it if required by a committer.

I think this is doable in the way shown in this patch. But this
seems somewhat bogus, too..

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center