Re: [EXTERNAL] Performance down with JDBC 42

Very good point from Danny: generic and custom plans.

One thing that is almost certainly not at play here, and is mentioned: there are some specific cases where the planner does not optimise for the query in total to be executed as fast/cheap as possible, but for the first few rows. One reason for that to happen is if a query is used as a cursor.

(Warning: shameless promotion) I did a writeup on JDBC clientside/serverside prepared statements and custom and generic plans: https://dev.to/yugabyte/postgres-query-execution-jdbc-prepared-statements-51e2
The next obvious question then is if something material did change with JDBC for your old and new JDBC versions, I do believe the prepareThreshold did not change.


Frits Hoogland




> On 5 Nov 2023, at 20:47, David Rowley <dgrowleyml@gmail.com> wrote:
> 
> On Mon, 6 Nov 2023 at 08:37, Abraham, Danny <danny_abraham@bmc.com> wrote:
>> 
>> Both plans refer to the same DB.
> 
> JDBC is making use of PREPARE statements, whereas psql, unless you're
> using PREPARE is not.
> 
>> #1 – Fast – using psql or old JDBC driver
> 
> The absence of any $1 type parameters here shows that's a custom plan
> that's planned specifically using the parameter values given.
> 
>> Slow – when using JDBC 42
> 
> Because this query has $1, $2, etc, that's a generic plan. When
> looking up statistics histogram bounds and MCV slots cannot be
> checked. Only ndistinct is used. If you have a skewed dataset, then
> this might not be very good.
> 
> You might find things run better if you adjust postgresql.conf and set
> plan_cache_mode = force_custom_plan then select pg_reload_conf();
> 
> Please also check the documentation so that you understand the full
> implications for that.
> 
> David
> 
>