Обсуждение: How to EXPLAIN statements inside a trigger function?

I'm developing some triggers for the first time, and I'm having
trouble analyzing their performance.  Does anyone have any advice for
doing EXPLAIN and the like on statements involving NEW?  For
instance, I'd like to know what plan PG is coming up with for this
fragment of a trigger function in PLPGSQL:

insert into allSenseRelationPaths (sensePath, weight)
select array[NEW.sense1ID] || p.sensePath, NEW.weight + p.weight
     from allSenseRelationPaths as p
   where (NEW.sense2ID = p.sensePath[1])
   and not NEW.sense1ID = any (sensePath)
   and NEW.weight + p.weight > minWeight
   and not exists (select 1 from allSenseRelationPaths as EXISTING
                     where EXISTING.sensepath = array[NEW.sense1ID]
|| p.sensePath)
   ;

The details of this don't matter much, but there is, of course, a NEW
record, as well as a local variable, minWeight.  I can't just
EXPLAIN ... this in psql.  I've tried constructing a regular function
as a jig, which sets up the variables, but I can't figure out how to
get EXPLAIN output out of that function.  I've seen this:

http://people.planetpostgresql.org/greg/index.php?/archives/106-
Putting-EXPLAIN-results-into-a-table.html

but that seems a bit heavyweight.  Surely others have wondered what
plans their triggers are using, how do you do this?

Thanks for any help.

- John Burger
   MITRE

"John D. Burger" <john@mitre.org> writes:
> I'm developing some triggers for the first time, and I'm having
> trouble analyzing their performance.  Does anyone have any advice for
> doing EXPLAIN and the like on statements involving NEW?

The critical thing you're probably running into is that the planner
sees such things as parameterized queries, and may not be able to
produce plans as good as what it comes up with for queries that have
simple constants in place of the parameters.

You can investigate what the parameterized plans look like with
PREPARE and EXPLAIN EXECUTE.  For instance, given your problem query

> select array[NEW.sense1ID] || p.sensePath, NEW.weight + p.weight
>      from allSenseRelationPaths as p
>    where (NEW.sense2ID = p.sensePath[1])
>    and not NEW.sense1ID = any (sensePath)
>    and NEW.weight + p.weight > minWeight
>    and not exists (select 1 from allSenseRelationPaths as EXISTING
>                      where EXISTING.sensepath = array[NEW.sense1ID] || p.sensePath)

you'd do something like

PREPARE foo(type-of-sense1ID, type-of-sense2ID, type-of-weight, type-of-minWeight) AS
select array[$1] || p.sensePath, $3 + p.weight
     from allSenseRelationPaths as p
   where ($1 = p.sensePath[1])
   and not $1 = any (sensePath)
   and $3 + p.weight > $4
   and not exists (select 1 from allSenseRelationPaths as EXISTING
                     where EXISTING.sensepath = array[$1] || p.sensePath);

EXPLAIN [ANALYZE] EXECUTE foo(... some realistic values ...);

Note that this isn't specific to trigger functions --- any
plpgsql function that does queries will have the same issues.

            regards, tom lane