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Thread: Issue with pg_get_functiondef

Issue with pg_get_functiondef

From
Edouard Tollet
Date:

Dear PostgreSQL developers,

I'm having trouble understanding the following, I apologize in advance if it is not a bug.
The following query works and lists the functions name and definitions as set in my database:

select * from (
    select proname, prokind, pg_get_functiondef(oid) as def
    from pg_proc
    where pg_proc.prokind = 'f'
) def

however, if I add the filter where def is not null, it returns an error:
select * from (
        select proname, prokind, pg_get_functiondef(oid) as def
        from pg_proc
        where pg_proc.prokind = 'f'
) def
where def is not null;
ERROR:  "array_agg" is an aggregate function

(with VERBOSITY set to verbose I get:
ERREUR:  42809: « array_agg » est une fonction d'agrégat
EMPLACEMENT : pg_get_functiondef, ruleutils.c : 2926)

I understand that somehow the query above is calling pg_get_functiondef on array_agg where I believe it shouldn't since array_agg has prokind set to a.
Filtering for pg_proc.prokind <> 'a' makes the query works again:
select * from (
    select proname, prokind, pg_get_functiondef(oid) as def
    from pg_proc
    where pg_proc.prokind <> 'a'
) def
where def is not null;

I get an error if I write:
select * from (
        select proname, prokind, pg_get_functiondef(oid) as def
        from pg_proc
        where pg_proc.prokind in ('f')
) def
where def is not null;
ERROR:  "array_agg" is an aggregate function

but it works (and lists both functions and procedures) if I do:
select * from (
        select proname, prokind, pg_get_functiondef(oid) as def
        from pg_proc
        where pg_proc.prokind in ('f', 'p')
) def
where def is not null;

I was able to reproduce these steps on a clean, up to date, Debian 12 install, without any data or user defined function.
SELECT version();
                                                      version                                                      
-------------------------------------------------------------------------------------------------------------------
 PostgreSQL 15.5 (Debian 15.5-0+deb12u1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit


Thanks in advance,
Best regards

--
Edouard Tollet
Data scientist | Stoïk

Re: Issue with pg_get_functiondef

From
David Rowley
Date:
On Wed, 13 Dec 2023 at 00:26, Edouard Tollet <edouard.tollet@stoik.io> wrote:
> I'm having trouble understanding the following, I apologize in advance if it is not a bug.

> select * from (
>         select proname, prokind, pg_get_functiondef(oid) as def
>         from pg_proc
>         where pg_proc.prokind = 'f'
> ) def
> where def is not null;
> ERROR:  "array_agg" is an aggregate function

The EXPLAIN output shows you what's going on here:

                                  QUERY PLAN
-------------------------------------------------------------------------------
 Seq Scan on pg_proc  (cost=0.00..155.49 rows=3124 width=97)
   Filter: ((pg_get_functiondef(oid) IS NOT NULL) AND (prokind = 'f'::"char"))

You might think you're forcing the prokind = 'f' to be evaluated
before the "def is not null", but the query planner has optimisations
built-in and is able to pull up the subquery into the top-level query.
The planner also thinks evaluating the qual prokind = 'f' qual last is
more efficient.

This isn't a bug... or at least if we were to "fix it", many more
people would complain as we'd have to do something quite heavy-handed
like stop pulling up simple subqueries.  Even something less
heavy-handed like evaluating quals from deeper (original) levels first
could cause quite significant performance regressions.

The best solution for you is likely just to add an OFFSET 0 to the
subquery. That'll prevent the query planner from pulling up the
subquery into the top-level query.

David

Re: Issue with pg_get_functiondef

From
Laurenz Albe
Date:
On Tue, 2023-12-12 at 10:33 +0100, Edouard Tollet wrote:
> I'm having trouble understanding the following, I apologize in advance if it is not a bug.
> The following query works and lists the functions name and definitions as set in my database:
>
> select * from (
>     select proname, prokind, pg_get_functiondef(oid) as def
>     from pg_proc
>     where pg_proc.prokind = 'f'
> ) def
>
> however, if I add the filter where def is not null, it returns an error:
> select * from (
>         select proname, prokind, pg_get_functiondef(oid) as def
>         from pg_proc
>         where pg_proc.prokind = 'f'
> ) def
> where def is not null;
> ERROR:  "array_agg" is an aggregate function

PostgreSQL evaluates the function before the WHERE clause.
Try this:

  WITH cte AS MATERIALIZED (
     SELECT proname, oid
     FROM pg_proc
     WHERE prokind = 'f'
  )
  SELECT proname, pg_get_functiondef(oid) AS def
  FROM cte;

Yours,
Laurenz Albe