Re: SELECT slows down on sixth execution

Jonathan Rogers schrieb am 17.10.2015 um 04:14:
>>> Yes, I have been looking at both plans and can see where they diverge.
>>> How could I go about figuring out why Postgres fails to see the large
>>> difference in plan execution time? I use exactly the same parameters
>>> every time I execute the prepared statement, so how would Postgres come
>>> to think that those are not the norm?
>>
>> PostgreSQL does not consider the actual query execution time, it only
>> compares its estimates for there general and the custom plan.
>> Also, it does not keep track of the parameter values you supply,
>> only of the average custom plan query cost estimate.
>
> OK, that makes more sense then. It's somewhat tedious for the purpose of
> testing to execute a prepared statement six times to see the plan which
> needs to be optimized. Unfortunately, there doesn't seem to be any way
> to force use of a generic plan in SQL based on Pavel Stehule's reply.


If you are using JDBC the threshold can be changed:

   https://jdbc.postgresql.org/documentation/94/server-prepare.html
   https://jdbc.postgresql.org/documentation/publicapi/org/postgresql/PGStatement.html#setPrepareThreshold%28int%29

As I don't think JDBC is using anything "exotic" I would be surprised if this
can't be changed with other programming environments also.

Thomas



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