Re: The same prepared query yield "-1" the first six times and then "-1.0"

On 8/21/23 08:27, Edoardo Panfili wrote:
> Hello,
> I am using
> postgresql version: 15.3 (Debian 15.3-0+deb12u1)
> org.postgresql.postgresql JDBC driver version: 42.6.0
> via Java 17.0.7
> 
> I discovered an unattended (for me) situation: when I execute
> 10 times the same prepared query the result is not always the same.
> 

> The attended result was a sequence of ten equal values but this is the actual result:
> p: -1
> p: -1
> p: -1
> p: -1
> p: -1
> p: -1.0
> p: -1.0
> p: -1.0
> p: -1.0
> p: -1.0

They are equal values:

  select -1 = -1.0;
  ?column?
----------
  t


> 
> All works fine if I open and close the connection after every single query
> but in production I am using pooled connections.
> This is what I can read in postgresql logs (it seems that after 4 queries
> the statement becomes named and the result changes after the second call to
> the named query):
> 
> 2023-08-21 11:51:50.633 CEST [1511] user@testdb LOG: execute <unnamed>: SET extra_float_digits = 3
> 2023-08-21 11:51:50.634 CEST [1511] user@testdb LOG: execute <unnamed>: SET application_name = 'PostgreSQL JDBC
Driver'
> 2023-08-21 11:51:50.644 CEST [1511] user@testdb LOG: execute <unnamed>: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.648 CEST [1511] user@testdb LOG: execute <unnamed>: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.649 CEST [1511] user@testdb LOG: execute <unnamed>: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.650 CEST [1511] user@testdb LOG: execute <unnamed>: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.651 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.651 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.653 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.653 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.654 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
> 2023-08-21 11:51:50.656 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name=‘first'
> 
> Can I do something to avoid this problem?

Read this:

https://www.postgresql.org/docs/current/sql-prepare.html

"
By default (that is, when plan_cache_mode is set to auto), the server 
will automatically choose whether to use a generic or custom plan for a 
prepared statement that has parameters. The current rule for this is 
that the first five executions are done with custom plans and the 
average estimated cost of those plans is calculated. Then a generic plan 
is created and its estimated cost is compared to the average custom-plan 
cost. Subsequent executions use the generic plan if its cost is not so 
much higher than the average custom-plan cost as to make repeated 
replanning seem preferable.

This heuristic can be overridden, forcing the server to use either 
generic or custom plans, by setting plan_cache_mode to 
force_generic_plan or force_custom_plan respectively. This setting is 
primarily useful if the generic plan's cost estimate is badly off for 
some reason, allowing it to be chosen even though its actual cost is 
much more than that of a custom plan.
"

> 
> thank you
> Edoardo
> 
> 
> 

-- 
Adrian Klaver
adrian.klaver@aklaver.com