Thread: Strange performance degregation in sql function (PG11.1)

Hi Andrew,

Thanks for your in-depth response.  I found that adding the stable qualifier didn't solve the issue unfortunately.  I actually encountered the same issue (or at least extremely similar) today and made a bit more progress on defining it. 

I have a carefully indexed and optimised query that runs in about 2 seconds with this structure and signature.

    create or replace function example_function(param1 int, param2 int) returns setof custom_type as

    $$

        with a_few_ctes ()

        select * from big_table where col1 = param1 and col2 = param2;

    $$ language sql stable;

This function works as expected when executed from a psql client.  I am calling this function via another framework (Postgrest) that executes the function using the following pattern:

    with args as (

        select json_to_record($1) as (param1 int,param2 int)

    ),

    output as (

        select *

        from example_function(

            param1 := (select param1 from args),

            param2 := (select param2 from args)

        )

    )

    select * from output;

Running this query with the args coming from the CTE resulted in my query running for many tens of minutes before I gave up.  Changing the underlying function to use plpgsql fixes the issue and the query runs as expected.  Both versions work as expected when called directly, but the SQL version does not when called with the args coming from the CTE as shown above.

The new function signature is

    create or replace function example_function(param1 int, param2 int) returns setof custom_type as

    $$

        with a_few_ctes ()

        return query select * from big_table where col1 = param1 and col2 = param2;

    $$ language plpgsql stable;

I haven't been able to check the bad query plan with auto_explain as the query doesn't seem to finish.

So to summarise, changing a stable SQL function to a stable plpgsql function when called with function args from a CTE fixes a huge performance issue of uncertain origin.  I hope someone can offer some suggestions as this has really confused me.

Best regards,

Alastair

---

From: Andrew Gierth <andrew@tao11.riddles.org.uk>
Sent: 19 May 2019 03:48
To: Alastair McKinley
Cc: pgsql-general@lists.postgresql.org
Subject: Re: Strange performance degregation in sql function (PG11.1)

 

>>>>> "Alastair" == Alastair McKinley <a.mckinley@analyticsengines.com> writes:

 Alastair> Hi all,

 Alastair> I recently experienced a performance degradation in an
 Alastair> operational system that I can't explain. I had a function
 Alastair> wrapper for a aggregate query that was performing well using
 Alastair> the expected indexes with the approximate structure as shown
 Alastair> below.

 Alastair> create or replace function example_function(param1 int, param2 int) returns setof custom_type as
 Alastair> $$
 Alastair>     select * from big_table where col1 = param1 and col2 = param2;
 Alastair> $$ language sql;

This function isn't inlinable due to missing a STABLE qualifier; that's
a pretty big issue.

Without inlining, the function will be run only with generic plans,
which means that the decision about index usage will be made without
knowledge of the parameter values.

Was your actual function inlinable? See
https://wiki.postgresql.org/wiki/Inlining_of_SQL_functions

You can get the query plan of a non-inlined function using the
auto_explain module (with its log_nested_statements option). The query
plan of non-inlined function calls is not otherwise shown by EXPLAIN.

 Alastair> After creating two new indexes on this table to support a
 Alastair> different use case during a migration, this unchanged
 Alastair> function reduced in performance by several orders of
 Alastair> magnitude. Running the query inside the function manually on
 Alastair> the console however worked as expected and the query plan did
 Alastair> not appear to have changed.

But when you run it manually, you'll get a custom plan, based on the
parameter values.

 Alastair> On a hunch I changed the structure of the function to the
 Alastair> structure below and immediately the query performance
 Alastair> returned to the expected baseline.

 Alastair> create or replace function example_function(param1 int, param2 int) returns setof custom_type as
 Alastair> $$
 Alastair> BEGIN
 Alastair>     return query execute format($query$
 Alastair>         select * from big_table where col1 = %1$L and col2 = %1$
 Alastair>     $query$,param1,param2);
 Alastair> END;
 Alastair> $$ language plpgsql;

Using EXECUTE in plpgsql will get you a custom plan every time (though
you really should have used EXECUTE USING rather than interpolating the
parameters into the query string).

I suggest looking into the inlining question first.

--
Andrew (irc:RhodiumToad)